I think Mustang would have won, only because he can keep his calm through battle and Ed goes a little berserk when called anything remotely sounding like anything that can be short. The experience in battle has a lot to do with who is going to win, but the emotion flowing is also going to be a big one as well.
You gotta take into consideration that Ed is the son of Hoenheim of Light. That dude's like one--if not, THE greatest alchemist ever. I'd have to say Ed, based off that fact. Yeah, Mustang's got him on battle experience, but Ed's got a greater understanding of alchemy than Roy does.
But with this information taken into consideration, you also have to take in the fact that BECAUSE Roy has much more battle experience, he'll know how to dodge attacks that Ed will throw at him. Also take in the fact that Roy is an adult, and Ed is only a child. No matter how much knowledge he can have in that little head of his, speaking in realistic terms Roy would win.
However, it'd be a very close call.
I agree with White Wolf,Roy would win because he's an adult.I am sure that when ed gets older though that he could beat him considering he's already a great alchemist now.
everyone gives roy 2 much credit, he may be expirenced, but he's still more of a desk jockey. he also has more than his fair share of hang-ups. Ed's no quiter Roy would have 2 come 2 close 2 killing him 4 Roy's liking 2 win