Should I use Kerdi membrane or cement backerboard?
First Kerdi can be installed on drywall or cement board.  However if installed on drywall it is only protecting one side.  The back side of the drywall fails and it is just as bad or worse than if it had an issue on the tile side.  

All shower/tub areas should have concrete board (I prefer hardiboard).  Whether or not you need a Kerdi membrane is a factor of use.  In normal residential use I wouldn't worry about it.  Backerboard plus a waterproofing membrane like Redgard is all that is needed for tub surrounds with thinset then tile.  Kerdi really offers very little advantage over hardiboard plus membrane.