Southwest USA is hard specially outside the bigger cities. Where exactly will you be staying. Most of the desert outside populated areas and away from major highways has crappy Cell phone service no matter what carrier you use
I think you would be better served by getting something that works where you winter.
Having had Virgin Mobile and T-mobile I think T-mobile has slightly better coverage where I go. Their plans are very flexible with the NO contract and monthly billing which allows you to change your plan at will, but the pay as you go is not as good.
The hot spot I got on EBAY or AMAZON for about $40.
http://www.ebay.com/itm/T-MOBILE-SONIC-4G-MOBILE-HOTSPOT-/371167786373?pt=US_Mobile_Broadband_Devices&hash=item566b52f585