This is one of those topics that's more important in sailing forums than in the real world.
It really depends on how you use your boat. If you're day sailing or cruising you won't feel a big "seat of the pants" performance difference between the two keels. On most boats you'd only see that difference if you're constantly tweaking your sail trim; something most cruisers and day sailors don't do because you're talking, snacking and enjoying the day.
You will be able to sail in skinnier water. Here on Barnegat Bay 18" can be a very big deal. We still bump bottom every so often, but the few times we've grounded I've been able to get her off without drama.
Now if you're serious about racing the fin is the way to go because that slight difference in performance is the difference between winning and losing a race.
Like I said it all comes down to how you want to use her.