Pick your carrier, go into the phone store, buy the phone you like. What's there changes all the time, more so smart phones, which are almost "phone of the week" if you must have the latest thing.
For travel, if I could I would choose Verizon, for having the broadest coverage with the fastest data services, though high speed data services are not so important if you don't use them. 3G, 4G data technologies have nothing to do with voice or text, while pictures, video, Internet access, navigation services do use data.
Nobody has service everywhere, not even voice, not even with roaming across networks. The law enabling mobile service made sure of that, intended to prevent creation of another communications monopoly.
Some people who have Verizon claim they've never been anywhere they didn't have service. I live where Verizon doesn't have service, voice roams to another carrier and Verizon is not licensed to sell service. My brother lives in a place where neither ATT nor Verizon has service, Nextel was the "national tier" provider and there were two "regional" providers providing most service. Mergers and buyouts helped close some of that, but if you want a national carrier, you have to buy the service for a home address where they are licensed.
Then there are the blank areas. I live in a place where I can get out or range of all towers for ATT, Verizon, Sprint, US Cellular, and T-Mobile with a 20 minute drive. Coverage is bragged as percentage of the population. You can cover 90% of the population of the US while covering less than 50% of the area. You can cover 80% of the population by covering about 1/4 of the area. Itis a big mostly empty country, and mobile providers don't like building towers to cover places where there are not enough customers to cover cost.
You are in Florida. You won't have many dead areas there, not where you can take a RV. The empty areas are desert and mountain wilderness, and the mpty expanses of the Great Plains and northern forests.
I carry a Tracfone for basic service almost everywhere. Tracfone is not a provider, it is a reseller. My GSM has worked in places where my ATT phone has not had service, although ATT is the primary carrier for the Tracfone. Tracfone contracts with other carriers in places where the carriers do not have roaming services.
On the other hand, my Tracfone does not work outside the US, my ATT has roamed internationally in Italy, Spain, Portugal, France, Germany, Austria, Hungary, Slovakia, and the Czech Republic. Voice, text, data. Same technologies, my phone has the bands, ATT has the roaming agreements. So it depends also on what you mean by travel.
