LB2120 4G modem AT&T / TMobile - SIM Card questions.

Trying to determine the best plan to use with the LB2120:

a) Prepaid xGB Data plan SIM card

b) Prepaid Unlimited Voice/Txt/Data plan SIM card

I looked at both AT&T and TMobile

Option a) seems to be more expensive at the carriers.

But I see cheaper pricing for b) however, I'm not completely certain this will work.

Can someone confirm that it is possible to use a "phone" SIM for the LB2120 and/or share experiences with using and setting up the service?

What do I need to communicate to the carrier.

Thanks;