Man, it's that fast. Wifi is pretty much the same though right? I'm using tmobile right now and the only thing killin me is the fact that I can't use 3G. Who knows I may upgrade later on down the line, but right now I'm good with my 3G.
There really isn't one. The only reason I'm doing it is becuase of the faster processor and RAM. Thats it. Apple has covered most of the flaws of the Iphone out, and these flaws have been corrected with Cydia.