I have stayed at both Key West Marriott Beachside & Marco Island Marriott very recently. They are both very good but also very different. The Key West Marriott Beachside rooms are much nicer than the standard room at the Marco Island Marriott. Night life is better in Key West but the beaches and resort amentities are not. There is a very small man-made harbor side beach here, and small pool. The fitness center is small too. The Marco Island Marriott is on an incredible wide beach and six mile stretch and it is beautiful with incredible sunsets. There are five restaurants, two huge pools, large fitness center, several watersports, and a large spa. Again the rooms are not as nice as the Key West Marriott Beachside. You can't go wrong with either choice. It depends on what you like (more night life = Key West) (more beaches & pools = Marco Island) Hope this helps
BigDogBill, is right on! Do you want a "Bizarre", time as in a "Monty Python" skit, then go to Key West. I really enjoy the food choices and the uniqueness of Key West. However, it would not be a place for all. I am not a "Sun lover". However, if you want "Peace", and do love the sun, then go to Marco Island. These two places are at different ends of a "Rainbow"!
As others have said, both places are great but very different. Another question to ask yourself is whether you want to do some sightseeing and drive to other areas. If so, Marco Island is better. It's a pretty drive from Key West to Miami, but not really a day trip...though it's doable. Personally, I'm also not wild about the crowds or the food in Key West...but that is a very nice Marriott.