If you have 3-4 days to spend in the Naples/Marco Island (MI)area, I would recommend staying a nice hotel in Naples (Ritz Carlton, Hyatt, Registry) and making a day trip to MI (i.e. skip staying actually on MI).
If you have a week, I would do 3-4 nites in Naples, followed by 3-4 nites on MI.
Why go for beach only (MI) when you can have the beach, shopping, & great downtown at Naples??? MI is nice enough, but personally makes me feel somewhat cut off from civilization (it's built up but mostly all hotels/condos...not as many shops/restaurants etc... as Naples).
Any further questions, just ask!