We've been to both with the last time being in 2019.
I had heard a lot about Jack's Shack so we made the 5 minute walk down the beach to it. We honestly thought it was overated, expensive, and filthy. The bathroom was disgusting and the service was horrendous. If you want some solace, their beach area is quieter and away from the cruise area. It's essentially a no frills shack on the beach. Won't be going back.
4/ 10
Margaritaville has the resort type feel. Large pool, swim up and stand up bars, food but expensive as well. Lot's of chairs inside and out and still close to the beach. Has a party atmosphere and some island vibe.
8/10