I currently travel for my job, and I'm usually on the road at least half the month. Wifi networks in hotels are good, and piggy-backing off of customers networks are better, but I run into the situations where wifi and piggy-backing (temp log ins) aren't available or too insecure for my needs. Currently I have unlimited data usage that was grandfathered in to the current plans, but will lose it next month. My work data usage varies or it would be a simple choice. Most months my usage is 2g or less using my smartphone as a mini-computer or hooking into a laptop to use as a modem for larger jobs as well as wifi/piggy-backing availability. But on other months when I have plans and blueprints, video conferencing, and continual contact with the home office, I can hit 20 gigs. The other thing I didn't add is watching netflix, facebook games, and video chats with family and that eats up data like crazy if stuck in a hotel with no wifi and poor tv choices.
I was looking at mellenicom (?) for their high data ceiling even though it would be pretty much wasted on most months, but the coverage availability map only covers the address and doesn't give you a general map so I can get an idea of where it is available. Also, I would like a service that I can hook up with a smart phone plan so I only have 1 payment plan, but am willing to split them if I have to. My company usually pays for phone and data contracts, but it is up to us field reps to chose our service based on our areas we work in, but they don't pay penalties on over-usage for data. Currently I work up and down the Mississippi river area from roughly St. Louis area down the New Orleans area so I pretty much am available to most carriers.
Most carriers now offer some sort of portable hotspot that converts the carrier's 3G/4G Internet connection into a local WiFi signal. I recently used a Verizon MiFi thing. Can't say much about it because I was so far out in the boonies that it could hardly get a 1X signal, let alone LTE. How much you pay for whatever you get; I guess you need to check with the carriers.
If coverage isn't an issue, I would go with t-mobile's $30 data plan. 5GB of 4G data before being throttled to EDGE speeds.. The plan won't work on hotspots or usb modems so you would have to get an Android phone and root it before you can start tethering.