I went with Sprint because T-Mobile is horrid in overall coverage. If you don't mind being stuck on EDGE, go for T-Mobile. At least Sprint launched LTE in my area quite some time ago (second batch of launch cities). AT&T has JUST launched it, and T-Mobile is still EDGE... Verizon, well... they've had it for years around here, but I'll NEVER go back to Verizon.
Sprint's service isn't exactly stellar, but we know this. I will say I've been satisfied with them so far. There are very few areas that I don't get service compared to my former AT&T plan. I also have fewer dropped calls. The Sprint Framily Plan is also contract-free, similar to T-Mobile. You have to pay off your phone if you cancel service, just like T-Mobile. I get unlimited data and annual upgrades. That works for me. On top of that, my service gets cheaper every time someone joins my Framily Plan until we're each $25 plus extras (from 7-10 people).
I think the merger could be a good thing, and it should help push Sprint closer to being a GSM network via LTE. I'm excited for Sprint Spark to get here. I've heard it's really good.