removing old flooring
P.S. you might want to read past posts on durastone they are having a lot of problems with it. good luck
I stand by Hardibacker. 1/4"
You can score and snap. And it's invaluable because it will not breakdown underwater.
You can screw it down or even put it down with roofing nails.
P.S. Don't go with the 1/2" unless you really need a strong subfloor. It will work, but you might have a hard time cutting it.