Hard to pit these two phones against each other because the speed and services are tied to the carriers. In that case in existing strong signal 3G service areas, GSM/ATT wins if iPhone is matched to it. Where there's 4G, Wimax/Evo combo looks more potent. I don't see it as a fair comparison if you put both phones in their respective best operating environment because that amounts to 4G vs 3G. I'm not going to say both phones are so far apart that they can't be compared, but either phone on Wimax vs 3G just isn't a fair comparison.
Perhaps it might be better to think of as which would you rather have than which is better. Then I think the lines become more clear. I'd rather have the Evo for all the reasons it's not iPhone OS- and ATT-tied. If another more powerful, 3G data-intensive device comes out on ATT, I'm just not sure how that network will handle it. Could be a different story if it was VZW, but I don't see Apple going against ATT exclusivity until LTE comes into play. ATTs off to a slow LTE start, and I think some unexpected, rapid rollouts will be a sign that they're in the running for the LTE-compatible iPhone.