Is it better to stay in Mackinaw City or Mackinac Island?

Definitely stay on the Island. Except for a water park and misc. shops and fast food places, there isn’t much to Mackinaw City except hotels and ferries to the Island.

How many nights should I stay on Mackinac Island?

While you *can* technically visit Mackinac Island for just a day from the mainland, I recommend spending at least 2 nights on the island in order to truly get the full experience.