iPad Air Wifi only or Wi-Fi + Cellular

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As far as I can tell, the Wi-Fi + Cellular model is the same for all the carriers and the reason you have to choose at checkout is because the iPad is shipped with that specific sim.

If I get a tmobile iPad Air in order to take advantage of their free data plan, will at&t LTE work if I decided I'd like more data (at&t is my phone provider)?

Does the tmobile sim just work right out of the box or do you have to set it up?

Is the $130 price difference worth it?