I just did a cross country trip to head to a funeral. The funeral was nice, but thats not what this post is about. Its about Verizon and AT&T mobile broadband, or more specifically Verizon mobile broadband (pre-pay no contract) and AT&T via tethered connection (iPhone). The setup was that I figured I could get some work done by using a mobile internet connection while on the road. I used Verizon on the way there, and AT&T on the way back. The results are that Verizon had much better speed in more places. That is very true, when I did have service, it was awesome. However it was very unreliable, constantly disconnecting whenever you switched cell towers. Also, vast parts of Kentucky and West Virginia had no service what so ever. AT&T via MyWi was a very reliable service (which is great for persistent connections), but it was dog slow. I got effectively dial-up speeds pretty much the whole way. I partly believe the unreliability of Verizon was due to poor software, but there is no way to tell for sure.
Which should you choose?
Verizon if you don't care about persistent connections and aren't traveling through Kentucky or West Virginia.
AT&T if persistent connections are required or your traveling through some dead states.
Or heck, you could do both, and use whatever is best at the time.