For me its a coin flip. Coverage where I live is equal, but I travel to New Hampshire, specifically the white mountains area and AT&T does not have 3G service there but verizon does.You need to choose the carrier that offers what you need.
Is this a show stopper for me, no but its one the small point to differentiate between the two carriers.
I've yet to call customer support with verizon wireless so I cannot judge that, but AT&T's customer support when I called them didn't suck. I wouldn't expec them (or verizon's) to be good but as I said they weren't bad.
Depending on a lot of factors, I'll either stick with vzw or go back to att. I'm not sure which but at the moment. At the moment, I'm holding off on the iPhone 4 just because we're getting close to the iPhone 5. I think I need an iPad more then a phone replacement and money doesn't grow on trees.