I think for the older iPads, there are 2 or 3 different versions with different LTE frequencies. They will work on other carrier networks but may not have LTE. But the new iPads got all the LTE bands for all the carriers. So they should work on all networks with the carrier sim card. The only problem I see is that Verizon usually don't let a non-Verizon device on their network. At least that's how they operate with phones... not sure about tablet. So the most flexible one to buy would probably be the Verizon one... since you get to change to any other networks later on if you want.

But the T-mobile 200mb/month 4G LTE + unlimited 2G over 200mb is really really attractive! I was going to buy the AT&T ones but now I am thinking T-mobile. If the network is really really bad.. I could always switch back to AT&T just need to get a sim. The free data would save me at least $120 a year per iPad. And I was going to get 3 of them. That's $360 saved per year...