It really depends on your particular travel style and need for HD and/or locals in regards to what will work best for you. Do you already have satellite TV at home? Do you even have a home, or are you fulltiming?
A permanently mounted dish of any type on the roof will only work when you have a clear shot to the geostationary satellites, which are approximately on the ecliptic. That means you will be in the sun if you want satellite TV. This will work for some, but probably more than half the time we are in the trees so that won't work obviously.
A portable solution with a long cable gives you more options in the trees, but even then it's not always possible to get a signal. The portable solution also means you will be carrying around a dish which could be an issue for some. The HD dish is big for DirecTV.
I can't speak for Dish, but the cheapest (not best!) way to go with DirecTV is the 18" round dish on a tripod, and a very cheap SD receiver added to your home account. You cannot get locals that way unless you are in a market that carries them in SD on the 101W satellite (which we are), and you have to be within the footprint for your locals, which only extends for a few hundred miles. If you don't have satellite TV at home, this probably won't make much sense financially though. Dish has a month-to-month option for RV'rs, but I don't have experience with it.
You might also consider whether you will have Wi-fi or 4G Cell coverage where you are going. If so, you can just set up a Slingbox at home and watch anywhere in the world that you can get an internet connection of more than about 1MB/sec. I like that solution way better than satellite TV, but obviously internet access is spotty at best.