I work for Sprint, and our coverage is good-same as Verizon's. They both work off of CDMA, and we dont charge for roaming (not sure if Verizon does or not). Nothing is locked out either. I use a BB Curve, but will be upgrading soon, to something with wifi as well.
The BB apps are great-I have bunch of useful ones installed to include weather, news, and live feeds. I have plenty of friends here who have ATT & their service is lacking. Now, ATT doesnt have a large footprint in my area, so I am sure that has something to do with it. But, if you are primarily concerned with staing in the US, I would suggest CDMA, as its more prevalent here. And, the newer BB devices have wifi capabilities, so you can surf & talk.