I think location is the most important factor when choosing the service. From what I've read, T-mo and Cing should have the exact same coverage in CA and NY/NJ areas. They co-own a third party company that maintains the cell sites in those states. I'm in Cali and coverage is very good. All other states, they're independant, but I'd assume they should be able to roam on each other's networks so if your T-mo phone has bad coverage, it's probably not going to be any better than Cingy's. I've had all four major service providers so here's my opinion on each....
Customer Service - T-Mo and Cingy were be far the best, Verizon next, Sprint last, horrible service.
Coverage - Verizon was the best although not significantly better than T-mo or Cingy. Sprint was the worst.
Value - By far T-Mo, especially if you require a data plan. Verizon was the most expensive.
Overall - Like I said, totally depends on your location, but for me, T-mo wins hands down. Sprint is the bottom of the barrel.
I'd take advantage of the 14 to 30 period most carriers offer before the ETA kicks in. Try out the service and see what works best in your real life situation. One more thing, be careful when changing plans mid-contract, Sprint will extend your contract date when you make a change, Verizon will not. I got screwed by Sprint on that one. Not sure what T-Mo and Cingy's policies are yet.