A sump is basically a box of water that increases the total volume of the tank, making it more stable. It is also a place to put heaters, skimmer, etc. so they arent cluttering up the main tank.
A wet/dry is a filter system that utilizes a nitrate production are to filter the water by either bioballs, live rock, etc. These can become a nuisance because they harbor nitrates which could give you high nitrate readings if not maintained properly. Most people end up taking the bioballs out and just use it as a sump.
IMO, a sump with some live rock and maybe a refuge compartment is your best bet. I am putting one on my new 120 and will attach a sketch below. You can make a sump out of an old tank or a rubbermaid container from wal mart. Do a search and you will find tons of ideas and plans. HTH