One or the other...
Neither tile, grout, nor cbu is waterproof and some moisture will get behind the tile. Now, not much and if you have the vapor barrier properly installed, it won't get into the walls. You don't want to trap what does get there inside.
Mildew takes three things to grow: the presence of the mold spore (almost universal), food (neither tile, grout, nor cbu is mold food), and moisture. If you keep the shower clean (soap scum, body oils, etc.) and it can dry out inbetween (good ventillation and not holding moisture), mold won't grow. It helps to seal the grout - it slows, but does not stop moisture absorbtion, and helps prevent stains from penetrating.
If you are really paranoid about it, you could tear it out and install Kerdi from
www.schluter.com. I trust it much more than Redgard. You'd need to redo the pan, but could leave the walls up. Check out
www.johnbridge.com.