it was a tough decision for me. I'm in the Bay Area. Everyone I knew used cingular because they had fair reception but were cheap, so they're parents got it for them. The rebels used ATT for the cool phones at the time like the Sony Ericsson T68i's. Everyone had some form of a nokia candybar phone or a motorola V60 clamshell one. The ones with cash went with Verizon and they're seemingly great reception/coverage.
I decided to go with Sprint. Why? I got my phone from Costco for one, and with they're great return policy, I can exchange phones anytime I want for free at no cost, and at the end of my 2 year contract if I want to change providers I can return the phone and get all the money back that I spent initially. Not to mention the Sprint rebate that comes with it. A lot of people said they have expensive plans. Well I'm paying $35 a month for 500 anytime, unlimited nights/weekends, and unlimited mobile-mobile, and I don't even use all those minutes. Lastly, people said they have the worst customer support in the universe. That, I can vouch for a little bit, because they screwed up my plan initially, but on a trip down to the Sprint store to look for a phone case, I resolved it in about 5 minutes.
The websites I gave will help a lot, dont choose your provider/phone without some research there. You DON'T want to be in a 1-2 year contract and get stuck with bad reception or a phone that you don't like.