Thin set or mastic... which is best for tile in a shower?

I have been told by a lowe's rep that mastic is best to use in a shower, but have read in a pro book that thin set with a latex additive is best...However, every thinset I have looked at says not to use in a shower? In addition, I am using 4x4 tile; every thinset says to use on 8x8 tile or larger...?? I am setting the tile over hardibacker on the walls and the shower pan is poured concrete, if that helps anyone give me the best answer. Thanks folks.