It depends on where you live.
T-Mobile coverage is worse than Sprint. In some markets you can get 4+ Mbps down on a HSPA 7.2 device with 2 Mbps+ upload speeds.
In other markets you can struggle to maintain a GPRS signal to make calls. I lived by an HSPA+ market before I moved where the service was like that. Awesome in the city. Drive 3 miles out and it goes from 3G to 1 bar of GPRS and I couldn't make a call longer than 30 seconds without it dropping.
The drive up here was terrible, because unless you're in a metro area the T-Mobile service is terrible.
I get full service here, but I've yet to get a Speed Test > 1Mbps down. Their 3G connection here is giving me high end EDGE speeds half the time.
When they're gone I'll simply go to another carrier. Given their service here Sprint and their Slow 3G speeds would be better than what I'm getting here (Spring has WiMax here, as well, and it speed tests like 2 Mbps down with 1 bar of 4G on the phone... ...)
In all honesty, I'd probably go with AT&T and get one of their LTE phones for two reasons:
1. Simultaneous Voice and Data - CDMA sucks with how it cuts off your data connection when you're on a call, and I don't like depending on the presense of both WiMax/LTE and CDMA coverage for that.
2. AT&T actually is working on expanding their network. Before I got T-Mobile I had no AT&T service. They told me to wait some months for a Tower to go up. I left cause I needed to make calls. Few months later everyone around me with AT&T had 3G/HSPA+ and I still had to walk out to the road to make a call, unless I used WiFi calling (which won't help you on a WP7 device, so their coverage forced me to keep the phone I wanted to replace when I bought the HD7...).
Sprint is cheap and they Have WiMax here so the SV&D is a bit less of an issue. However their phone repetoire is kind of bad. I'd basically be limited to an Android phone on them. Verizon is expensive and is a show stopper. AT&T offers the most choice IRT devices and platform choice.
Also, the FCC would never allow Google to purchase T-Mobile, not after having purchased Moto Mobility. That's just asking for trouble. It's like if Microsoft had purchased HP's PC devision.