I've done a bit of experimenting with plans/carriers over that last 2 years, and my observations are as follows.
don't know anything bout sprint, but I've never heard anthing but complaints with them
verizon is overpriced period, and unless you are a jetsetting roadwarrior traveling salesman, where their coverage map is of utmost importance, brings nothing to the table to justify their price
t mobile, on paper, is/was the best overall value, as far as their plans, my faves goes, however after going from att to t mob, and back to att on 900/1000/900 min plans respectively, you are not getting your full minutes, I garauntee you if you make 900 minutes of calls on att, and 1000minutes of calls on t mob, you have signifigant overages on your t mobil bill trust me, their "minutes" are more like 30 seconds
Ive never had issues with att coverage (obx, middle of chesapeke bay, etc), they now have 5 favorites (like t mobs 10 "my faves") which is nice, and finally rollover minutes. $ for $, min for min, att works best, for me anyway.
now for data plans, I did the smartphone thing for awhile, and its a joke, I mean calls, voicemail, email, browse, what else do you need, I've yet to prepare a word doc or screw with a spreadsheet , on a tiny friggin screen, copy and paste whoopty friggin doo don't need on on phone. Now the same obligatory $40/mo smart data plans that you get when you upgrade your device is the same friggin $15/mo data plan for all other non smart devices, same product, different package and price. take your smart phone and swap out sim cards with a regular browse plan, guess what, works the same. I did exatly that with my last smartphone till the phone was trashed.
better yet have regular $15/mo data plan on regular phone, and teather it via BT to a palm, ipaq or your laptop and viola, your back to having all the functionallity again, only on a better screen, at half the cost. I'm typing this right now from my laptop that is teathered to my phone via bt, no diffeent than wifi, 15/mo