I was thinking the smaller of the two 1/2" drive impacts, the "compact" one. The bigger one has more torque, but I was thinking more for swapping rears, trans, or other awkward places, that the little smaller package would be better.
I did buy a new 3/8" adaptor for the impact driver I got. Those disappear more frequently than anything else.
I will say that I wasn't very impressed with the battery life. I bought the "brushless" ones, they looked a little better built and are supposed to get better battery life. I ended up using my grease gun battery because its a lot bigger battery. The two batteries they sold with the drill/driver package were the smaller ones, and they aren't up for driving a bunch of holes.