If you are just going out and back in one day, take the resort tour (especially if Felipe (sp?) is the guide). It gets you to the keys about noon and doesn't leave until after sunset. We went in Jan. Had lunch, toured Truman's Little White House, toured Hemminway's home, watched the sunset from an outdoor restaurant, and then had time to catch some of the street performers. Got back about 11pm.
We had been to the Keys some years before for a number of days, but this was still an enjoyable trip. Much too long a drive for me to do in one day in my car.
If you haven't been out there before, the tour would give you a chance to see if it is a place you might want to spend more time at in the future.