I personally prefer the look and strength of metal shelf A/V racks. Glass is good too. I like the look of glass, and might use glass if it wasn't for the number of 50-60 pound components/amps I have in my A/V racks and the 60+ pound center channel w/ built in sub. With those components to deal with, I felt better about the metal shelves for my equipment. One guy who had the same center I do (CLR 3000 by Def Tech) had a glass shelf break that was either above or below his CC can't remember which. I'm sure there are many factors that made it break, and it was probably a freak accident, but that made me think twice about putting my CC on one.
I think you're right about the high humidity over a long period being a possible issue for you. Glass or metal may be your best bet. I'm not a fan of a lot of MDF shelves. I'm sure there are some nice ones, but I like the look of my metal Bell'O racks. I've only had a couple MDS shelves, and that was 10 years ago, so I'm sure they are nicer looking ones now. I change out some of the gear in my rack fairly often (amps mostly), and I'd probably chip the MDS shelves.