Purely based on observations cruising Ft. Lauderdale waterways in a skiff in the days following Wilma, I would have to say safer on land. I saw very few boats with no damage in the water. Most had relatively minor damage but many were badly scarred or worse.
The boats that I saw with damage on the hard were badly prepared, ie sails and canvas left on, not strapped down to ground
anchors, stands not chained together.
All boats are vulnerable to flying debris but the consequences to a boat on the hard will never be seawater submersion unless the storm surge is phenominally high, in which case boats in slips will be over their docks and pilings anyway.
If you can afford both...? If you love the boat dearly and don't want to be left with an insurance check than you are likely safer on the hard, properly prepared (encourage or help your neighbours to do the same).
I hope others will offer their observations as well but those are mine.