There are typically 9 seconds difference in the handicap ratings under PHRF. So, if you race a lot and the the depth of water in your harbor and sailing ground is not an issue go with the deep keel.
If you are a cruiser, it roughly translates into a 15 minute difference on a 100 mile trip, everything else being equal (and some more tacks on an upwind beat).
If you sail in shallow water, go with the wing. Most of what you read about the difficulty in getting a grounded wing keel off the bottom does not apply to the C-320 if the bottom is soft. If you have decent sails, they point very well and are fast. Where I sail there is lots and lots of shallow muddy bottom, we have the wing keel and love it. Saturday, I had to do a bit of aquaculture by plowing a furrow out of and back into our slip...no big deal, except to the bottom paint.
Deep keels are more prevalent on the west coast as they have lots of deep water.
In the Chesapeake, NC estuarine systems, FL and Caribbean, wings seem to dominate due to the close proximity of the bottom.