The "Dolphin Encounter" (which is interacting with the dolphin) is a very limited experience. You get to spend a few minites on a platform where the dolphin swims by, you pet him, he gives you a kiss and that's about it.
The "Dolphin Swim" (the belly swimming experience) offers more opportunity to interact with the dolphin in the water. You can pet them, swim along with them, they will take you for a ride, etc. At least those have been my experiences in both Cozumel and Nassau.
In my opinion - it is worth the extra money. While both excursions are not cheap for the experience I think the "Encounter" is overpriced.