Verizon has the most complete network. Sprint has a roaming deal with Verizon so if you have Sprint you have voice coverage everywhere Verizon users do.
Data coverage is another story....if you are using an air card or similar, Sprint still roams on Verizon, but Verizon pushes roaming users down to 1xRTT which is slower than dial up. If you are looking for good 3G/4G coverage it has to be Verizon...we are switching back to them when our Sprint mifi contract is up next month.
AT&T now has the fastest data network with more 4G coverage, but Verizon's is overall more extensive with the most 3G/4G coverage.