Although I use Verizon, and that it would be awesome to have the iPad on Verizon, I would rather have it on AT&T.
Verizon=more 3G coverage, slower network
AT&T Not as much(about 2/5 the coverage), but faster network.
Plus, you can't switch carriers with Verizon, seeing as they use CDMA, and CDMA doesn't use SIM cards.
People don't see the truth. AT&T does have a smaller 3G network, true. But their speeds are faster than Verizons'. Most people online say that they have awful reception. They happen to live near huge metro areas, meaning they are not the only one accessing the bandwidth. People living in areas which are not as crowded should be OK.