An unlimited data plan is great, but having a reliable, usable connection when you want it is priceless. No single carrier covers the whole area you might want to travel to, and if you're thinking about going to Canada or Mexico it gets even worse. And you can't really rely on the carrier's coverage maps to make a decision on who to go with.
There are many places throughout the US and Canada (and probably Mexico) that offer free or really inexpensive WiFi. I found in my travels last year the the best solution was to set up a schedule where I logged in to check and pay bills once a week and then look for a WiFi spot for that activity. Where I spent most of the summer there was no cell coverage with my phone but the local Laundromat had free WiFi, so bill paying day was laundry day. :-)