The problem with using a 15 day money back guarantee if traveling is you won't cover all your locations within the 15 days. I spent 304 days on the road during the 2013-2014 winter, most of the time in the southwest & western states, much of the time away from cities and main roads. I carried both a Verizon based hotspot & an AT&T phone, both in Sleek amplifiers.
While the coverage along major highways & cities was pretty much equal between the two carriers, once I got away from civilization the Verizon coverage was better. Of course there were exceptions, and if you are going to stay in one location most of the time it pays to choose the carrier that best covers that area. If you are moving from place to place without prior knowledge of carrier coverage, & planning to spend most of your time away from cities & towns, I'd choose Verizon.