Well I always prefer to use more then 3/4" ply even tell the home owner thats its better when I do installs just for peace of mind. You can get away with 3/4" over 16 oc with ditra but not over wider joists. If you have the room to put 5/8" then 1/4 or 1/2" then I would suggest that.
You also can leave osb and lay 1/2" bc ply over then ditra.
You also can leave osb and lay 1/2" bc ply over then ditra.