I know the dilemma, I was lucky enough in 2009 to do both in my August vacation.
I had a 7 night Magic cruise, 9Was a Eastern went to a Western re bad weather). That was good.
But we also had a day at Discovery Cove. We got there at 7.30am. We had a private cabana booked. We were checked in by 7.45am, there was a queue at this time. After photos taken and charge cards given, we went to the Cabana, and had our orientation at 8am and in the pool with Dolphins at 8.20am! That was nice it was cool, but not cold, it became very hot then rained later.
We had the most fantastic dolphin experience, I have swum with Dolphins with
DCL at Cozumel, Costa Maya, Acapulco, but this just put thoose into pale memories against this.
We had breakfast therefore 'after' the swim, not normally that way around.
We then had an uninterrupted day to enjoy the Cabana, with great views over the Dolphin area and all of the other swims, and drinks, and snacks service included, as well as going for lunch.
We went around the lazy river, snorkel pool, etc,
Both are A1 experiences its hard to decide.
Looking at your posts you have a deal at GF, so do that and DC this time and cruise next time.