I have a tethering plan on my iPhone 5s through AT&T and in the past, my iPads have all been wifi models because I could tether. But, I went ahead and splurged and got an iPad Air with cellular this time.
Main reason is so that I can just have the option of another carrier if I'm in an area where AT&T is bad. The air came with a Verizon SIM, so if I'm traveling and data is bad on AT&T, I'll just fire up the Air on Verizon and do mobile hotspot the other way.
Also, the cellular models of iPad Airs have GPS. Wifi models don't.
Main reason is so that I can just have the option of another carrier if I'm in an area where AT&T is bad. The air came with a Verizon SIM, so if I'm traveling and data is bad on AT&T, I'll just fire up the Air on Verizon and do mobile hotspot the other way.
Also, the cellular models of iPad Airs have GPS. Wifi models don't.