I'll try my best. These are all "informed assumptions," if you will. I based my choice of carrier on this line of thinking, which is why I eventually went with Verizon.
CDMA:
Verizon - true nationwide footprint, entire footprint is 3G, but their 3G speeds are not as fast as the 3G speeds of AT&T/T-Mobile. So, their data is more consistent, with fewer drop offs, but doesn't have the top-end speed. Their 4G plan is to have their entire footprint covered by 2014.
Sprint - a nationwide carrier thanks to roaming agreements with Verizon. When roaming, you're limited to 2G speeds, and 250MB/month roaming data (unlimited in-network roaming). 3G speeds are similar to Verizon, and available in most areas. First to roll out 4G service, which is WiMax through Clear. Up in the air how much they will expand 4G, or if they switch to LTE.
GSM/UMTS:
AT&T - true nationwide carrier, but their network is hodgepodge. Urban areas and more highways are served with 3G, but areas outside of the drop off to Edge, GPRS, or even no data (just voice). Their 4G plans haven't been fully announced yet, but they will likely just do the drop-offs again, leaving only urban areas with 4G coverage.
T-Mobile - nationwide carrier thanks to roaming agreements with AT&T, has 3G/"4G" coverage in fewer areas than AT&T, more areas with drop offs to Edge/GPRS/no data. Their 4G plan thus far has been to speed up 3G technology and refer to it as 4G, rather than invest in true 4G technology.
If I just wanted a phone for voice, I'd go with the cheapest carrier. But as for data coverage, Verizon has the broadest (area wise) coverage. It's also all 3G and planned to be converted to 4G. But, if you know for a fact that the areas you travel to have good coverage with AT&T (from your current/prior use of them), then you should be good on just about any carrier.