Booster seats (for the car)

[hapahaole76]

So Kaylie is almost too big for her infant seat. I've got to figure out what to put her into next. We have Jack in a Britax Marathon. So I could give the Marathon to Kaylie (and hope it doesn't take up too much room rear facing) and get a booster seat with 5 point harness for Jack (one of those ones that are convertible where you can take the back off down the road and just have a little seat). Or I could get another Marathon or another seat for Kaylie and keep Jack in his Marathon.

Any thoughts or recommendations?