i think the question is a little open ended.
are we talking about homebrew blades? large sized hombrew blades? small commercial blades, or large scal MW+ turbine blades.
i'll concentrate on smallish hombrew blades.
i think the best medium is whatever the builder is comfortable with using, as well as what assets the builder has in his/her shop.
sure, if you've got a three or four axis mill, vacuum bagging equipment, and the knowledge on how t use it all, fiberglass is probably better.
however, if you own a saw, a chisel, a sander and a paintbrush, wood is probably far superior for that particular builder.
for my blades i kinda split the difference. i carved them from laminated wood blanks, then put a layer of fiberglass over the top. then coated liberally with two part paint. 1.5years up, and they still look like the day they were installed!
adam