I know this is an odd place to start, because the astronauts and cavemen fight is all about Spike and Angel and how they deal with the hole in the world and strangers versus loved ones and zzzzzzzzzzzzzzzzzz I dozed off there for a second. Boring, and far less apt. Because Wesley’s involvement in the emotional and theoretical argument - do the astronauts have weapon? - shows himself as exactly the pragmatic, modern, logical mind that moves the tide of humanity away from the cavemen and turns some of us into astronauts. Angel, necessarily betwixt and between, romanticizes the astronauts even as his physical, animated argument with Spike puts him closer to the cavemen than either of them would admit.
The comparisons and sharp, illustrative contrasts between Wesley and Spike begin long before the two characters meet. They’re wildly different types of awakenings, but Angel’s reunions with his respective gangs in Innocence and Epiphany have similarities distinct enough to be intentional and thought-provoking. The mirroring of Drusilla and Cordelia - the dark lover-sister Seers created by Angel’s mission, whether for good or ill - stands next to the visual and story echoes of Spike and Wesley. Angel, now and forever, thinks of himself as better than Spike and Wesley, Called and Chosen and Cursed and whatever else have you, but of course at that moment in both stories (well, I think always, but, contentious o/t argument) they are well beyond him in whatever mission he’s claimed. Spike, not Angel, is the ferocious vampire warrior known as the Slayer of Slayers. Angel needs his epiphany because he’s abandoned the helpless, where Wesley has never considered doing such a thing. They’re both far ahead of him when he goes back to them, and he knows it even if they don’t.
In order to re-unite Angel with his group, both shows remove the ability to fight, with Spike and then Wesley as temporary wheelchair users (both due, I add, to Angel's failures - to kill or at least exile Spike and Dru in early BtVS S2; to stop the zombie cops in time in TTDL). The exceptionally problematic use of common disability tropes aside, the two characters represent the leaders of the life which Angel hopes to re-join, with his brute strength emphasized by their altered mobility.
Because of their histories, Angel is responsible for them, in very different ways. Not just accepting Wes onto his team way back at AI, or accepting the cat Drusilla dragged in (but Angel oddly does accept these strays, whether because his Daddy Issues compel him to beat his father by becoming one, because he sees their potential, or because he is in his own way as lonely and desperate for companionship as they are). But he’s been incalculably influential on their minds. You were my sire, man, you were my yoda! Drusilla made me a vampire, but you made me a monster! The mind-wipe. All Wes experienced at AI was because of Angel, and much of it was for Angel - yes, including Connor, especially Connor; saving the baby is also sparing Angel what Wes thinks is the inevitable and unthinkable, the thing that would destroy Angel finally and utterly.
They occupy a similar space in relation to Angel, chosen by family (Wes being born into a Watcher family; William chosen by Drusilla) but, unlike Angel, they consciously chose to make themselves into the fighters they are. Spike is Angel’s vamp side, Wesley his humanity. They take what’s Angel’s - leadership of AI, you always were the brains of the operation, Buffy, the vampire with a soul - and oh, yes, his children. Jasmine and Darla gave him Connor, but Angel made Drusilla, thoughtfully and obsessively, and Spike beat down a lackey and took a sleeping Dru exactly the way Wes did Connor. Just in case we missed it, Spike’s job in NFA is exactly the one Wes attempted in S3, to save the baby.
Even the Shanshu isn’t necessarily Angel’s with Spike around. Spike and Wesley together render Angel himself unnecessary. There’s nothing that is just Angel but his destiny. And that isn’t his either. It’s the Senior Partners’ plan for him. You can have everything, but there’s nothing that’s yours. Spike and Wesley conceive of Angel’s PTB-given status as exalting him over them, but their non-Chosen status is exactly the thing that lets them choose their own paths.
I’ve talked before about Wesley’s building and shedding of personas and identities which are selective, heavily censured facets of a consistent core personality. The only character in the ‘verse who does this better than Wesley is, of course, Spike. In both of their first appearances they’re in a strikingly similar persona. It’s probably happy accident, but if you watched the episodes in the order as aired, GWBG comes right before FFL. The FFL/Darla two-parter is about the history of the Fanged Four and how it affects the present. But the present is always the focus, and just before the two-parter we get a peek into William, through Wesley. Check out William the Bloody Awful Poet, and then Wesley Wyndam-Pryce, Field Watcher. I defy you not to see it. Spike’s ragging on Wesley - “Percy here used to be called Head Boy”- is pretty much the only light moment in Lineage, but as we know, it’s also a joke at Spike’s own expense, as we’ve met young William and know he was just as much of a Percy as Wesley, though probably less successfully.
Our two shape-shifters start out in painfully similar skins. Of course, it’s as much a persona as anything else is, but they both have the social stations and personalities which would allow them to choose exactly that exterior, not least of which an instinctive ability and need to create and adopt a persona. It’s not just the poncy exterior, either, but the reasons behind it - the raw, badly-cloaked emotion, the eagerness to be recognized, the warped and misaimed but unmistakable potential, striving overzealously to live up to society's rules. Those core traits lead to the serial façade-building.
They are of course not the same - Wesley is a general, Spike is a brawler; Wesley is an academic, Spike is a poet; Wesley is Order and Spike is Chaos. In short, Spike is fire and Wesley is ice. Yet even their very opposition evidences their existence on the same plane. (After all, what is S4 Spike - evil but chipped, killing demons to sate his blood-lust, rapid-cycling through alliances with Adam and Buffy- but a rogue demon?)
And though the personas they choose and the abilities around which they build these personas are different - Spike is an exceptionally powerful vampire, Wesley a magically and intellectually gifted human, and this means that they must choose different personas - they both evince a similar level of discomfort with these personas, just enough to remind us of the person underneath. Spike goes almost comically overboard in his Slayer of Slayers persona, adopting a “second skin” to remind us that he’s not the same underneath. He then blunders as an Ineffective Bad, and spends three seasons chipped before he goes off to create a new souled self. Wesley the demon-hunter trips over his own two feet, and his desperate certainty that he is deeply dangerous.
Wes and Spike are the characters who are explorations of free will, both determined and revealed not by their destinies but their choices. Spike isn’t good because he has a soul, but because he could and did choose to get a soul when he lacked any reason to believe it would benefit him. Wesley isn’t the sharp, distant fighter that he becomes because he kidnapped Connor; he could and did lay waste to everything he held dear because he was always willing to do what he was convinced needed doing. But for all that, neither of them are defined solely by their potential, but by the massive consequences of their decisions. Spike remembers his crimes before his soul, re-lives them afterward, and is dragged back from the beyond to fight with his past embodied by Angel and Dana. Wesley, not once but twice, gets and loses a reprieve from memories of his gray phase, the first from Jasmine, the second from Vail.
It’s not that their minds are even wired particularly differently, so much that they express themselves in such different ways. Wesley is always the rational harm-reductionist, the forward-thinker. He researches, he plans. Spike is the id to Wesley’s superego, but once they are within a conflict, they’ll show remarkably similar reasoning skills - pragmatic, resourceful, and ruthless.
The two beat their fathers in succession, Destiny occurring only within weeks of Lineage, the astronaut with his gun (do the astronauts have weapons? yes, always, you can’t go into space without tools) and the caveman his bare fists. Both battles are in a sense hollow, as neither Roger nor the cup was real, but that’s what allows for true victory, as they’ve both proven to themselves that they can move on. They are the characters who grow and change, and defeating their pasts is part of that, which the final killing of either father figure would not allow. Darla: Your victory over him took but moments, but his victory over you will last lifetimes. (The Prodigal) Having killed his own father, Angel can never defeat him, and will circle the drain for eternity. Spike and Wesley are linear; they can move on as Angel cannot.
Wesley and Spike are the characters that were closest to Fred at the time of her death, and the ones who dive into understanding Illyria. Spike dives into figuring out her strength; Wesley can neither look at Illyria nor look away from her, so he collapses behind his books and tries to understand her mind. They can be led astray into the mindless slaughter of NFA because they’re an aimless head and hand, dependent on a contemptuous god-king of a dead world for their heart. They stare at each other across the imperious blue hole in the world until they both fall into it.
Destroyed and re-created, tossed out onto the street more than once and springing back only to be stronger and more useful than ever before, they must pass each other on different planes over and over again. They build and shed personas in the way that they do because they are choosing not just what to do, but who to be. And that is where the astronaut and the caveman, for all their differences, cannot help but level, because at its core, that is the most human choice of all.