Those are the two different network technologies used by the U.S. carriers (essentially). T-Mobile & ATT are GSM/UMTS/HSPA+/LTE carriers, while Sprint, Verizon are using CDMA/EVDO/LTE. Virgin Mobile is only using CDMA technology AFAIK, and they are on Sprint's network. You might find some helpful info in the following thread:
It doesn't sound like you have a choice except going with the carrier that works best where you need it at all times. If that requires two prepaid phones, then that's what you apparently may need to keep doing.
I have both a Tracfone and a Virgin Mobile prepaid phones. I had Verizon but it I did not get good reception where I live. I do most of my work at home and needed reliable service. I get that great reception with both Tracfone and Vmobile. However, I sometime must travel into an area where nothing will work except Verizon. I, of course, get very little or no service at all. What can I do? Does either of these carriers sell a phone that will work in a Verizon service area. I've read that G at the end of the model @ means it is a ATT network and that C means Verizon??? What does that mean... Any help will be greatly appreciated.