As long as the board is 1/2" cement board and mudded, let the installer pick the material he's most comfortable working with. All you might want to concern yourself with is that whatever he chooses, make sure YOU read the warranty to see that it's an approved substrate. Just ask the builder for a copy of the warranty, say, you want to just read it. Or, if you know the mfgr of the tile, contact them directly for a copy of the warranty.
FWIW, a stiff frame is 2x10 14"OC with a 3/4" plywood subfloor. Cement board on top of that is stiff. My best to ya and hope this helps.