your location definitely affects what the "best" service is. I have 8700g and I bounce between Orange County and Los Angeles almost daily, and I have had people in the biz of selling cell towers tell me that Verizon is the best in OC for service, but from experience, Verizon isn't great in parts of LA (pacific palisades/Malibu) - I have Tmo and I get good but not perfect service in OC and great service in LA.
I'd talk to your friends about their coverage, but also look at Tmo's website because you can look up what their coverage is in your area or nearly any other location.
Tmo appears to have the cheapest data/phone plan but cingular gives roll-over minutes, both have advantages and drawbacks.
If you really need voice command, you could always buy a bluetooth equipped Prius (or put a similarly equpped head unit in your car), and sync it with your phone