Speed means nothing. It's a handset device, you only need a few megabits max anyways.. The real key is latency (how long your request takes to propagate). If you look at those speed tests you'll probably notice that ATT has higher latency, at least here in Columbus, Ohio where I'm at. I consistently get 2500+ms ping times on ATT, but 160 to 180ms on Verizon.
From articles I've been reading lately there has been claim that ATT has their towers set to buffer high amounts of traffic due to congestion, unfortunately this has a side affect as its not allowing TCP/IP to do its job. Which is probably why I am having so many issues with browsing the web and using network apps lately. I'm coming from a Blackberry Bold and will say that ATT service has gone down hill here lately in terms of reliability, hence my main reason to switch. My girlfriend has the IPhone and she is having issues as well as of the last few months. Rather disappointing as I've always hated Verizon due to their bastardizing of their phones, but you have to admit they have a good network.