REal thinset is a cement based product that cures into a rock-hard adhesive. It comes dry and you must mix it with the specified liquid (often just water). Mastics (sometimes they're incorrectly labeled thinset!) are a glue, pre-mixed in a bucket. They need to dry verses a thinset curing. The thing with a mastic is if it is placed under a nearly impervious tile (like say a porcelain or glass), it may take forever to dry out; and, if it's in a place where it can be continually wetted, may NEVER dry out. Until it dries, it can move around. Also, in the fine print, there's often a size limit on the tile - this again is because the stuff needs to dry, and since it normally does that through the grout line, a bigger tile limits the escape path for the moisture. FWIW, I had some mastic get on a piece of wood trim once. After a couple of months, I was able to wash it off, since water just softened it. Not something you want to use in an area that can get wet. Another FWIW, there isn't much difference in bond between the smooth and rough sides of cbu...don't worry - if you apply the thinset properly, it will stick to either side! If the cbu is being installed on a floor, make sure to use thinset UNDERNEATH it, per the manufacturer's instructions. This is to ensure you get 100% support underneath the cbu. Typically, when used on a wall, you'd put a vapor barrier up on the wall before installing the cbu (in say, a shower) unless you are planning a surface waterproofing material.