I stayed at the Renaissance Marina tower for a week last month. I loved it, but this was just my wife and I. No kids. I think the Ocean Tower at the Ren would be better with kids, as others have mentioned. In either case, the two are located about a five minute walk from each other and you have access to all the facilities.
My one point of contention with the Marina tower was that the rooms were quite small, but I really loved the Renaissance. It felt very neopolitan, if you know what I mean. I like the fact that it felt small, as opposed to the monstrosity that is the Marriott on the other end of the island. (The Marriott shares space with two Marriott Vac. Club properties and is quite crowded. I stopped by there during our visit. Also met a fellow platinum member while sightseeing who echoed this sentiment). And the Renaissance is downtown, in close proximity to shopping and restaurants, thus you don't necessarily need a car. Seems to me the Marriott is sort of in the middle of nowhere.
The private island was worth it to me. As opposed to the above poster who said the boat ride was inconvenient, I found it quite fun to take a speedboat ride twice a day. And the beach at the island is very secluded, with a ton of shade trees, thus eliminating the need for huts, etc. This was a big, big plus. Of course, there's also a decent beach located at the Ocean Tower as well, if you're not up for the boat ride on a given day.
One other thing, there are about a dozen or so eateries -- some upscale, some coffeeshops, and everything in between -- located at either tower where you can bill stuff to your room. Most are pretty good, and heck, it's more Marriott points!