Engadget's got a good review and comparison at http://www.engadget....on-iphone-review/

What I found interesting on the speed test is while AT&T has significantly higher throughput, the latency is also much higher. But then this is nothing new. AT&T's HSPA data network was always known to be faster than Verizon's EVDO overall, it's the coverage that's vastly superior on the Verizon network.