Switch them both, it's not that big of a deal. I went from a high wing trainer to a low wing tail dragger (a 'low wing trainer'). My only instruction was to 'fly it!' so that's what I did.

Between the two mentioned, get the Cherokee. The cub is more of a floater, and if you're into warbirds you should have something a bit faster and more sporty, you will need the practice!

RCKen has his famous list of planes that will surely pop up in this thread. My second plane was the Phoenix Dolphin, $99 from Tower and a lot of fun. The Cherokee is supposed to be a good second plane; I flew a Hangar 9 Cherokee (also $99) and it was really easy to fly. It was a buddy's plane and I flew it while I was still flying my trainer. It was actually easier to fly than a plane with tons of dihedral.

Good luck. This is where the fun starts, in my opinion