 Re: Refugium or Wet/Dry
I'll be the first to tell you what I know, Then the experts will tell you all the right stuff  As far as I am aware there are two (2) ways to look at a refugium.
1.) Refugium is meant to be a "refuge" for plants and other critters big and small that you do not want in your main tank. This "smaller" tank is either hung on the side of your existing tank or placed below your tank like a wet/dry. It gets its water from the main tank thru a gravity drain (overflow) and has has its own lighting system and pumps to return the water back to the main tank.
2.) The other use or thought is to put it down below like a sump and to use it as a filtration system for your main tank. It would contain maybe a filter pad, lots of plants, macro algea and other things to help breakdown the ammonia, remove phosphates and other harmful biologicals. It could contain fish or inverts but it is mostly setup as a filter type system again with its own lighting and pump ssytem to return the water flow to the main tank.
Read some of the threads about refugiums and you'll see what I mean on the different schools of thought.
I just realized I didn't answer your question about the advantages or disadvantages. I'm sure someone else will. I'm just getting the thread started * 
Happy Halloween,
Jim
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90 Gallon glass, 135#'s of rock and 2" sand. 2-250w 20k XM metal halides. 2-95 watt actinic VHO, 4 powerheads, Coralife skimmer. Maroon Gold Striped Clown, Blue Chromis, Kole Tang, Pygmy Angel, Serpent Star, Brittle Star,Gorgonians, Mushrooms, Yellow Polyps, Turbo Snails & Blue Legged Hermits. 20 gal sump with Caulerpa, Mag 7 return pump. | 
