I just base it off this, and this opinion is in the BOSTON area. Service may vary depending on your area

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Anyway, I have an iPhone 4 on Verizon, my brother has a 3GS on AT&T. Since I'm a geek and made him download speedtest.net, I was shocked the see that he was receiving downloads of 3.6 mbps

!! My downloads were only .70 mbps!
Kind of angry, I decided to do a little test. I opened Apple.com, Google.com, nytimes.com, and macrumors.com on both phones. I made SURE that the CACHE WAS CLEARED, HISTORY WAS CLEARED, AND COOKIES WERE CLEARED ON BOTH PHONES. Both phones has 5/5 bars, with AT&T's speedtest murdering Verizon's.
Anyway, as I loaded up each website, the Verizon iPhone managed to beat it every single time. To me, it doesn't make a lot of sense, but Verizon's network still managed to beat it by 1-4 seconds every time I tried it. That's why I don't care much about speedtests anymore. PING IS WHAT REALLY MATTERS HERE. If you have a low ping (verizon) then the web pages will start loading instantly. If you have a high ping (at&t) then they will start loading slower.
I also based it off of coverage. Most of the time, my brother has pretty good service, he will have 3-4 bars, I will also have that. But there are many times where he randomly drops all service to none (dead zones) which he and myself found annoying.
Overall, if your in the Boston area, I would recommend Verizon. It really depends on where your located. Location is key. We brought these two iPhones to many different places, and the Verizon one was the overall better choice. Again this is from my testing, your's may vary. I just wanted to point out that download speeds don't tell the full story, ping does. If you have a 200 ms ping with Verizon, and a 500 ms ping with AT&T, the Verizon iPhone WILL load faster and vice versa. Location is key.