I'd reccomend the P-51.
Why?...........
Because from what I read, you really want to improve your skills, not just look like you are. The P-51 is going to be less forgiving of some mistakes than the Edge or the Extra. The Extra and Edge are awesome planes that will let you power out of a lot of bad situations. You will probably crash the P-51 more than you would either of the other two, but you will learn how to "fly" it, as opposed to learning how to power out of mistakes. Just my opinion.
