Well we love Marco Island, spent 10 days there for the last two years
Naples will be our next choice (when not staying in friend's apartment on Ft Myers beach

), especially the Registry Resort.
I think this is the important thing. Where will you be staying - in a hotel/resort or villa/self catering.
We feel like we've "done" Marco now - but would not hesitate to recommend it for anyone's first time down south on the gulf coast. How many of you are going, children, etc.
The beaches on Marco are great - we stayed at the Marco Hilton, very nice, fab location on the beach, nr restaurants, and Naples is only 16 miles up the coast. You will get hooked on the Floridian's national pasttime of shelling!
We don't tend to spend all day on the beach when we go to the gulf coast each year, a few hours in the morning before it gets too hot and then wander off for a look around (if I wanted a beach holiday I'd stay right where I am on the south coast here in my beach hut) - Florida has SO much more to offer than theme parks and the Naples/Marco area is just teeming with things to do, or nothing at all other than lazing on a beach if that's what you like (like my DH

).
There's the Everglades, boat trips, mini-golf, shopping (both
WalMart and the more upmarket designer shops), there are day trips to Sanibel/Captiva, Ft Myers, you can explore the Everglades for days and see different things - you have to take in an airboat ride. There are loads of different restaurants.
Sorry, got me on my fave subject..... give me shout if you can think of anything else you might look into.