I think your Extra should be better than the other 2 you have ready to fly. I base my opinion simply because it is the only one glow powered so it should be the only one that is light enough to fly very good, assuming good servos are used. That said, I would choose the Extra to compete. The other two can be good choices or can be very good back-ups...

Back in 1999, when I started flying IMAC (basic) I flew a GP 300S 40 size. I did not win so much, but had a lot of fun. In 2003 I was in the need of a better plane and built a Cap 232 25% glow powered. It is resonably light and has lots of power. It flew intermediate in 2003 and advanced in 2004. With that plane, I made only 1 contest without being in the top 3 (it was a really windy day, when smaller planes really get a big disadvantage).

I say all this to show you that the proposed sizes of plane you have are good. Don't worry too much about the size of the plane but more about the quality of the components of the planes and quality of your practice time.

Get a good caller (my girlfriend is the best caller I ever had) and practice with what you have. Don't change your plane until you feel limited by the plane.