If you don't travel overseas then the best plan is the one that has the best coverage in the places you do go in the US. If you do travel or have plans then you may want to give ATT greater weight since the ATT version is unlocked. Also ATT iPads are easier to sell than VZW ones, precisely for this reason. Note, easier to sell does not mean they are worth more, just easier to unload when you are ready to sell.
The ATT and VZW pricing is fairly similar so I would not let pricing be a determining factor. It has to simply be based on practicality.