I'd use the cbu...no mold food, won't degrade if it does get wet. You could apply a surface waterproofing like RedGard if you wish. CBU will absorb moisture if it gets to it, but will not degrade. Make sure to use the special mesh tape on the joints. You can apply this while tiling, it's for strength, not for show like on drywall - you don't want a hump there, so installing the tile while it is in the soft/pliable thinset prevents 'speed bumps'.