I agree with bruce on this point. I started flying a Tower Trainer .40 MKII ARF this spring, and I've been very pleasantly surprised at how aerobatic it is considering its flat-bottomed wing. I think have a shallow dihedral is at least as helpful as a semi-symmetrical airfoil. The Arrow .40 has both. The Avistars generally have less dihedral than a lot of the other trainer ARFs that are on the market.

A flat-bottomed trainer can still do a wide assortment of aerobatics, at least as long as it's wings aren't sticking out like bunny ears like on my Nexstar.