I've been really happy with T-Mobile. Works seamlessly from country to country and no extra charges for using data or text. You'll need to see what your reception is like at home though.
Certainly dumbfounded by US Cellular, and not negatively memorable last summer. Did use the 310 for gpx files though, vs. Strava (and left it home this time). But access to data for nav w/o excessive cost is essential.
Have a voip setup waiting at home and believe it offers some reasonable benefits/possibilities. Traditional landline needs to go ($77/mo for now mostly unwanted, unsolicited calls) but our cell reception is unsatisfactory.
I wasn't very happy with Verizon's international plan. $25 for each 100mb of data, and you go through that pretty fast if you use google maps to navigate around. (I used 350 mb for a 15 day trip.) Eventually, however, I realized you could use wireless to download the map before you go out and then navigate yourself using the map with cellular data turned off.
Still some of the "Wild West" of cell phone contracts, "deals" and rip-offs :-)