It's likely to cost you a little more to go with the smaller tiles because the gunite needs to be floated behind the tile (more acurately) for the smaller tiles. This additional labor costs more. With the 6" tiles the irregularities can be hidden better - they just glob on more motar behond the tiles to level them but with the smaller tiles the wall surface needs to be smoother/level before the tiles can go on. I also like the smaller tiles because they make raised wall section look taller.