The big consideration would be If you want HD or not. DirecTV's HD signals are carried on KA Band ONLY, most in Motion RV Antenna's are KU Band only. To get an in motion KA Band solution you could be talking $5000 to $7000 and up. All of Dish's channels, both SD and HD, are carried in KU Band. KU Band in-motion systems start in the $2000 range.
I have an In-motion Intellian Brand KU Solution for my Boat and it works great, I use it for DirecTV, picture quality is not great on DirecTV's SD channels, but I get my Local's (Via 119''s Spotbeams) as well as all of the Core SD channels. My system can support upto to 2 Satellite receivers but Receiver one controls which satellite I'm pointed too. So If I had two receivers, Receiver #2 would only be able to get either my SD Locals (when receiver one is receiving SD Locals) or Core Service (When receiver #1 is tuned to Core Channels).
As stated above most KU Band Solutions can support both DirecTV and Dish Network Service.