I have stayed at both, splitting our time between the Marriott and the Marina Ren. The Ren is actually divided into 2properties, the Marina and the OceanFront Suites. The huge benefit to the Ren is the private island, it is spectacular. The beach at the Marriott pales in comparison IMO. Oceanfront suites has larger rooms, and a neat outdoor restaurant. No casino, but you can walk across the street to the one at the Ren. The spa is truly great, too. Comparing everything, when we return we will only stay at the Suites.
been to both in Aruba, I like the Renaissance best, they have the private island and a better pool. Now the Renaissance is two hotels in one. If you want to be closer to the water, the Ocean Suites is the place to stay. The casino is in the Marina hotel but they are next door to each other and there is a casino between them.