As long as you are aware of the weight differences (20cc vs. 20cc, 30cc vs. 30cc and 20cc vs. 30cc) I don't know how you could go wrong with either? If I had to pick, the 30cc's might not be quite as picky on their needle settings? To me though, the more compact air frames used with the 20's make them friendlier from a handling (hanger rash) perspective, and are easier to transport. If you're as cheap as I am, it shouldn't be hard to see the 20's are a little less money as well - engines AND airframes...