Depends how much you and your son want to do motorized water sports. GP doesn't include anything motorized, but as mentioned, seadoos and parasailing is readily available at extra cost anywhere on the beach. The difference in cost between the 2 resorts would probably allow for a lot of motorized rentals. It is entertaining to lounge on long bay beach and watch all the people, locals, tourists, strolling vendors and musicians. As mentioned, Alfred's next door has live music about 3 nights a week. If you're up for it, you could take him to the jungle for dancing. I don't know how much he'd need to be entertained, but at his age, I'd think he'd prefer live reggae bands over Beaches entertainment geared for kids.

Beaches would be a lot more flashy than GP, so comparing would be difficult. I think you need to try to let go of comparing properties and realize that you now have different needs and this is a new adventure. (or not, if your needs remain the same) Depends on your son too. My 17 yr old is not the "Beaches" type. He has long hair, plays the guitar and isn't terribly active. For him/us, Beaches would be a waste of money and hanging out on long bay and visiting the cliffs is way more his thing (as mine).