I use to have AT&T, I went to the areas they claimed to have HSPA running, or 3G.
I have tried using speedtest in Denver, Houston, Salt Lake City, SanFrancisco, and Verizon was far superior in speed and signal. Unless you're running a dedicated data card, the AT&T 3G speed never got above 700kbps. My Droid speedtests at 1.4mbps in rural areas, same in metro. Tethering my Droid to a laptop yields the same bandwidth for the laptop.
I can also use more than 2GB without paying extra...... regarding the comment about not doing voice/data simultaneously.... wouldn't it be nice if your iPhone did voice at all?

I had a 3GS and in the 6 month time period I burned about 1500 minutes...and of that I only had 2 phone calls that didn't drop.