We did the Blue Lagoon Dolphin Swim many years ago. Looking at videos of the island recently, I regret not staying longer and enjoying the beach time included.
I have been to the Atlantis for the water park area.
My recommendation: check out the rest of the property for both to help you decide. Make a day of it. The dolphin activity will probably only take 60 minutes or less, include preparation.
And check out the difference between Dolphin Swim and Dolphin Encounter. One is treading water the entire time, the other is standing in water. Is it the brief foot push or is it petting and interacting with dolphins that is important?
Go to the websites for both places - they both give a pretty clear idea of what you experience. Don’t assume you get more than what they spell out.
Are the parent(s) planning on participating or just watching? Costs can be a factor.