16" centers and 3/4" subfloor is about minimum. T&G is always better. I'd top any wood substrate with 1/4" hardi backer board in a bed of multi-purpose thinset (ANSI 118.4) and screw it down to stiffen it up. Use Schluter Reno U or other ramp shape to get rid of added height if it causes problems outside the bath area.

Tape all joints with 2" mesh tape and thinset.

If it was my house, I'd use a trowel applied WP membrane over walls and floors before setting tile. Something like 2 coats of Custom Bldg Products RedGard or Laticrete HydroBan. Waterproofs and also acts as an Isolation membrane. Both are easy to use and work well. Tile is adhered directly to the membrane surface.

Numerous wall boards are available. 1/2" hardi (diff to screw), cement boards (wonderboard or Durock), DenSheild products (vinly coated gyp bd), and good ol green board. All work. Most require a vapor barrier lapped down to the tub flange or WP over the top to keep water out. DenSheild you can caulk/tape joints becasue of the vinyl face.

__________________
Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)