You can use either or. They do make a mastic for inside the shower.
If it were me (or at least what I have done in the past) is to use thinset. If water gets behind the grout (at it probably will some day) it wont loosen anything up.
One DISADVANTAGE for thinset is that when putting tiles into the shower you need to go from the bottom up. Thinset doesnt really have that great sticking capabilites. If you put a tile up in the middle of the wall it will slide or fall down. (yeah it happened to me)
Mastic on the otherhand has great sticking capabilities. I just trust it to keep the tiles on for the long haul. (I've had it fail in one of my showers)
If you havent put the walls up yet, you might either consider wonderboard (cement board) or hardiboard (composite myster material)
Hardiboard is MUCH easier to cut. You can score and snap it. With wonderboard its much more difficult to get nice clean cuts (unless of course you use a mason blade on a circular saw (just makes allot of dust))
Hope this helps.