Personally, I always believe the most important consideration first is coverage. Regardless of how you feel about a carrier, you have to go with what will service you best.
At&t is starting to get a good selection of devices, and it will probably get better in the months to come now that they aren't the only Fruit stand in town.
No one can tell you what you need and what is truly important to you. Myself, I could care less about the mobile coverage, I am on Wifi 99.9% of the time, so whether my mobile experience is on Edge/3G or no G, doesn't really matter to me. It is more important that it acts like a phone when I am mobile and can send/receive emails, although knowing I can browse or use Google Maps when I need to is nice too
I went to T-Mobile for a short period of time when I got made at At&t (don't even recall the issue). It didn't last long as I was constantly roaming after I moved.
Good luck and welcome to the family.
And yes, rooting allows you to delete all the crapware and load other apps onto the device.