Looking at the pic. Are the tiles on a wet bed? So you want to smash out the wet bed too? They look higher in the pic?

If your committed to scrabbling out the tile bed then Battens down on the slab. I am in over 500 houses a year mate and walk on plenty of floating floors, I stamp my feet in every house to determine structure and Battens on slab sounds solid mate. No way I would screed that.