a wet/dry works like any other filter...helps convert ammonia to nitrite, nitrite to nitrate. wet/dry's are particularly effective at nitrite to nitrate because of the 'dry' stage allowing extra oxygen to keep the bacteria thriving.
A protein skimmer using tiny bubbles to trap DOC
's..which are both attracted to and repelled by water due to having 'poles' of electrical charge, much like magnets. The surface of a bubble has water on 1 side, adn not on the other, so it's a magnet for DOC
's means you'll have less stuff converting into ammonia, nitrite, nitrate, as well as phosphates.
I started my 40gal with a Prism skimmer. (not a good skimmer!) I had nasty problems with cyanobacteria, typically caused by low flow (not in my case) or high phosphates (which I did have).
Within 7 days of upgrading to a CPR bak-pak 2R, my cyano
disappeared, and has never returned. I did nothing else to the tank, not even a water change, so I know the skimmer worked it's magic.
If I were you, I'd get the skimmer, and a little more cured live rock (or cleaned dead base rock) and use it as you primary bio-filter.
If you go with the CPR, look for a site that sells them with maxi-jet pumps...or read my thread in the Hardware forum about upgrading to a maxi jet. It's cheap and really makes the skimmer work better.