Fast downloads mean nothing when you're on a tiered data plan. Especially now that these smartphones are becoming more advanced and faster.
Personally, I would go with wherever you can get an unlimited data plan. If you're still on your unlimited data plan with Verizon, I would stay with them. I've been with AT&T since I got my 3GS and will be staying with them since I can keep my unlimited data plan, but if I couldn't, I would go to Sprint.