Having played both Mass Effect 2 and Bioshock 2 I found them an interesting contrast.
ME2 is supposed to be the "role playing" one, with "choices" and "character development". But frankly having played the bioware formula before all the choices were obvious and the rails, though in theory branching, were obvious as well. I never identified much with the character, since making good or evil choices pushed up the good/evil meter which had game effects, thus pushing back on your choices. The whole story didn't pop. My favorite moment was getting the scientist to sing a version of "Modern Major General".
And the action sequences? Pretty bad really. The use of the cover mechanic meant that any fight was immediately preceded by a whole bunch of waist high cover cubes, which take the surprise right out, and the enemy AI was incredibly basic.
Overall they took out a lot of the useless stuff from ME1, but also took out the interest. If you've played any bioware game ever, you've played a better version of ME2.
Bioshock 2 is supposed to be the shooter. And it's true that your character is not that developed and the story is totally on rails. That said, I identified with the character much more than in ME2 anyway. You don't get many choices, but I found myself making them thinking about what felt right. And the background of the story is much more interesting - they hit two different ways that no-holds-barred libertarian capitalism can fail.
It's not as ground breaking as Bioshock, and maybe a bit shorter, but I liked it almost as much anyway. Like ME2, they took out some of the useless stuff and that was good, but unlike ME2 they left in the good stuff. Combat on regular was not that hard but not dead easy, definitely harder than ME2 even though you get effectively infinite lives in Bioshock 2 if you like.
Plus when you set someone on fire in ME2 it does some damage, big whoop. In Bioshock 2 they run around screaming and try to douse themselves in water. Also, Bioshock 2 has the gorgeous setting of Bioshock, but with more barnacles. What's not to love?