The best carrier for you will depend on where you are and where you go. I had Verizon and loved the service but my house and a few other places I frequented were black holes for Verizon, so I switched to AT&T/Cingular and life was good again. However, a friend also has Cingular and his house is a black hole, sounds like he's on Mars.
For data I think Sprint offers the best deal, at $15/mo. Cingular (if you get their MEdia Net Unlimited plan) is tied with T-Mobile (package) at $20/mo.
If you use Sprint's employee referral offer you can get a nice phone plan and unlimited net for free! You may have to get the Treo from them to get that, maybe not.
http://www.sprint.com/sero
Use
joe@sprint.com as an employee email address, enter your zipcode to get you into the deal section. I just checked and the Treo isn't showing anymore for my zipcode. Perhaps yours?
I'm not recommending Sprint to you, or any other carrier. Just suggesting that maybe asking around to see what works best for you in your areas is a good way to go. Here, a few places I have a hard time with T-Mobile is rock solid.
As for Earthlink, they use Sprint and Verizon networks from what I read.
Good luck!