The difference between AT&T's and Verizon's customer service is definitely like night and day.
I've decided to swap my Verizon iPad for the AT&T version today. I spent $30 for 2GB of Verizon data and went thru it pretty fast, however, mostly to experience the novelty of LTE. I think for me, the ideal price point is the 3GB AT&T plan for $30, so therefore, I'm going to switch. I'm also hoping to get a faster LTE connection. With Verizon I only got around 11 Mbps. I'm envious of those getting over 20, so if the AT&T version gets me that, I'll be really pleased.
My biggest regrete when I switch from Verizon will be the that the iPad will be harder to resell when I upgrade. I think the Verizon version will be more popular, primarily because they have a bigger LTE network. As for tethering, it isn't an immediate need and will like be offered by AT&T in the near future. Needing more than 5GB a month was also something I considered, but that will be rare if ever.
It's a hassle to go back to the store and doing the back-up restoration, but if nothing else, swapping the iPad re-sets the 14 day return window. And testing both companies LTE networks personally, is really the only sure fire way to know which is best for you.