I think with 4 days left you may be limited by which one is available.
If both are available they are very different experiences.
Cape May Cafe is a buffet - serve yourself. It has a focus on Seafood, but has many choices for non seafood lovers as well. Many people rave about the Crab Legs. It is all you care to eat. We were not overly impressed, but certainly didn't hate it.
Captain's Grill is an airy sit down restaurant. It has a set menu and you will be served at your table. It is very low key and relaxing. We think the menu has a bit of something for everyone, and we enjoyed it.
I think the Captain's Grill is more likely to be available on short notice that Cape May Cafe.
Enjoy