I do agree with Bossman. HOWEVER, it would be good to hear some confirmation about WEST COAST USA coverage of T-MO prepaid. Chicago is NOT West Coast. Neither is Richmond, VA.
OTOH, it is clear that the original T-MO strategy in US was to start in the big metropolitan areas and major interstates and then expand coverage.
The OP should also check the T-MO USA PREPAID (NOTA BENA) PREPAID coverage site for cities or areas to be visited. The states mentioned are large and there is not 100% coverage, especially rural and desert. I don't have that link right now.
www.t-mobile.com should be close enough. There is a separate prepaid coverage map which is thinner than the postpaid.