I actually just compared it to a friends speeds on AT&T, he's using the international GS2. Around here they are pretty equal, he averages about the same between 1-3 mbps down. My upload speeds were actually always higher than his.
So basically it seems like if you have the international version you will get pretty much identical speeds with Straight Talk or AT&T. But you save half the money, which definitely seems worth it to me.