Can't speak to the Marriott...but I stayed at the Renaissance last September for a week long stay. It's nice...very nice. Great location...tons of shops and restaurants within 30 seconds walk. Casino isn't bad either...they take American dollars in addition to local currency. The private beach is cool...and from what I hear the sand was brought in from Aruba...lol. Parking garage right across the street was convenient and free.
They treated me like a Platinum should be...upgraded room with a view and balcony, even though I paid for the cheap room (was on business). I remember the internet being overly priced, as you'd expect...but that was my only complaint.
We've been to the Marriott twice. The latest was last year. Visited the Renaissance then to make a comparison. For us, the Marriott is superior. We liked the natural beach front. The snorkeling right out fron is among the best on Curacao. On our last visit (for points) I requested a room upgrade when I made the reservation. When we arrived the manager was called out to personally greet us and tell us all the latest goins on there and on the island. They also gave us a beachfront room!. all we had to do was open our outside sliding door, take about 10 steps and we were on the beach. We had several wonderful evenings watching the sun set right from our patio while enjoying a glass of wine.
The Renaissance is right on the harbor and in town. If you like that setting and the man made beach on the 2nd floor, then you should go there. The beach did look nice, but it was very small.