I can always go with green board (as in sheet rock not brew board)!
Greenboard isn't that much better than regular drywall, from all that I could tell when researching for my kitchen. I used it by my sink just to handle a little bit of extra humidity (straining pasta, for example), but really, if it's an application where moisture is an issue you use cement board (which unfortunately isn't what you're going to want as a finished surface).
If you do use the plastic, I'd think about putting it on top of a cement board base, since it's completely immune to water damage. Belt and suspenders, I'd personally hang 1/4" cement board and then glue the plastic (Liquid Nails) onto that.