With AT&T you can walk into a store and they'll hand you a SIM no problem. Already done it, but didn't get the right one for prepaid the first time (you CAN activate it, but the correct one is pre-activated and just needs funds added). No questions or anything either way. Just a SIM and a receipt with $0.00 on it and out the door. Didn't even ask for my AT&T account info, so it's not just because I'm an AT&T phone customer.
I had Verizon years ago, and my biggest fault with them was their atrocious customer service and downright nasty store employees. I had almost no signal where I worked, and if they'd just LET ME OUT OF CONTRACT. Instead of giving me a bunch of phones, and making me spend hours on hold for nothing repeatedly, I might have came back to them. Instead, I eventually (my Alltel account got bought) ended up on AT&T.
Verizon's coverage has got much better than it was (it's similar to what Alltel's coverage was back then - ca. 2005-2006), but AT&T has made massive enhancements and the coverage area is much wider than Verizon's. All HSPA+ which blows away Verizon's EVDO (though to be fair, Verizon should have LTE by sometime next year here). HSPA+ is more than adequate with the caps though, and AT&T's caps are more generous.
And, best of all, with AT&T, while a couple people have been a bit clueless or had attitudes, 95% of people who work for AT&T are amazing. Hold times are almost always within a few minutes (vs. 30 minutes to an hour for Verizon, at least 6-7 years ago), and the network runs so much better.
I don't understand all the AT&T hate people have (even people IN MY AREA). I mean seriously Verizon feeds their customers something. I can have great HSPA+ signal on my phone, be surfing the web doing what ever, and watch a Verizon friend (one specific friend... but there's others like him) unable to do anything with his phone and curse at it. When I told him "shoulda got AT&T" he said "well, Verizon's better almost everywhere else, it's just this spot" all defensively. But it's not. There's very few places in Montana where Verizon's better (though there are some).