I had a GP Ultimate ASAP for one of my early planes. Flew great, lost it too early, battery came loose.

The problem with most second planes is short life. I'm going to suggest you spend a LOT of time with that trainer - at least 200 flights, working on challenging stuff each time. Work your way up to precision routines, including IMAC maneuvers and inverted flights. Then the chances for your second plane are pretty good.

Nothing wrong with a biplane, if you set it up correctly and take your time getting to know it, for a second plane. I'd start with an Ultimate, something that's popular and easy to set up. Get used to the taildragger differences before you take off - taxi a lot.

Good luck,
Dave Olson