This shouldn't come as a big surprise, considering Verizon's EV-DO Rev. A maxes out at 3Mbits, whereas AT&T has deployed HSPA+ to almost all of its network already, maxing out at 14.4Mbps (in theory), which helps its 3G devices download speeds as well. Plus, the results largely coincide with our own data speed testing on the two iPhone 4s we did recently.
The issue, though, is usually not speed, but coverage and network load, so now that you have excellent smartphone offerings on both carriers, it all depends how well you are covered where you live or travel by one carrier or the other, or whether their cell phone towers are enough to handle the pressure in the city you live in. We are including screenshots of the carriers' own current data coverage maps for reference purposes below.