I'm voting Verizon simply for the fact their coverage is just too good to ignore. I'd love to be on T-Mobile because it's hard to ignore how cheap it is to be on their network, but their coverage is just not good outside of cities. The frequency T-Mobile uses also has issues when it comes to reception within buildings. There are a lot of people who love the carrier, but even plenty of the loyal ones will say the signal isn't the greatest by any means.
If you don't plan on traveling and your city has good coverage, it's fine, but if you do travel, it's a more difficult decision. I hate the cost that comes with being on Verizon, but it's nice getting data in areas where ever other carrier fumbles. In fact, at the job I'm working part time at right now, I'm the only one who does not need to use the wifi there because I get great 3g reception.
Also, I think the G2 is going to have a better dev community than past LG phones. This is the first flagship LG Android phone to release on all carriers.