Marco and Sanibel are quite different. Marco is more about boat trips, dinner cruises, fishing, wave runner tours, eco tours, etc. Most of Marco is private, so beach access is limited, unless you stay at a resort on the beach. Sanibel and Captiva have more public access to the beach, but the public parking areas get parked in quite early during season. Ding Darling is no big deal - if you go to Google Earth and look at the entire southwest Florida, there is nothing special about that space, other than nobody can build on it. Consider being a migrating bird, and from the air there is no attraction. There is MUCH more open space and wildlife down by Marco. If you do go to Captiva, look into the boat ride to Cabbage Key and get a cheeseburger in Paradise. There are many nice restaurants on Captiva, but you will have to make your own fun. Many people suggest 1st timers go to the Bubble Room for dinner (I'm sure you would like the 50's toys under glass in the tables, the Bubble Scout uniforms, and the trains running around the ceiling - get the grouper in a bag! DELICIOUS!), but we like the Green Flash and Timbers. Also, we consider Captiva to be one of the most beautiful beaches ANYWHERE, but there are no shells there. Sanibel has lots of shells, but I always considered Marco the best place for shelling. If you decide on Marco, you will need to know how to find the only 2 public beach access sites - South Beach and Tigertail Beach. Best way to see Marco - get a room at the Marriott JW.
PS: During season it can take an HOUR just to get from the causeway from Ft Myers to Captiva. It's a LONG 2 lane road, and every time somebody wants to make a left turn, you're STUCK. We used to take day trips up there, but we have rarely been back since they increased the bridge toll to $6.
In my opinion, San/Cap has not been the same since hurricane Charlie