A wet/dry filter is where air and water trickle on bioballs,bail.ceramic rings to break down ammonia and nitrite(hence the name wet/dry).Another benefit of wet/dry is a higher oxygen level.
Refugium is a place to cultivate macro algae and copepods away from predators they may consume them(hence the name).They also is use as a living filter,with a deeper sandbed,macro algae,live rock,and anerobic bacteria can go a step further and consume nitrate and phosphate.(BTW,macro algae releases oxygen as a by product so it can add oxygen also)
One more thing,,,what are your plans...full reef,fish only,or fish with live rock only?