I am a more laid back person and staying at the Marriott was too much hustle and bustle for me even though the accommodations are very nice. I prefer the Ren.
Just a thought but can you break your stay into two stays and try both? That is what we did last time with a stay at Lacabana (Eagle Beach) in the middle. It gives you a taste of the two different areas (Palm Beach and downtown). That way you get to try both and know what you prefer for next time because there is always a next time in Aruba!
Both places are very different. Having stayed at the Marriott Hotel and only visited the Renaissance Island once, these are my comments:
The Renaissance Island is very nice, peaceful and plenty of chairs. However, if you like to walk the beach and participate in any watersports activities this is not the place for you. The location of the hotel is in a great downtown location and lots of things are very walkable. Again, personal choice of where you like to be.
As far as the Marriott goes, in the early part of December it is quiet so getting chairs will not be an issue. The hotel is lovely and located at the end of the beach. Lots of beach to walk and lots of water activities available. So much has been built within walking distance of the hotel like restuarants, malls and casinos in all of the other hotels. It is part of the "strip" and at night there is lots to see practically in your backyard.
Very personal decision and it really depends on what you like. However, I do have to agree with Cindy that the boat ride is very nice and pleasant going to the island. We really enjoyed our day there. However, our kids are watersports people and Palm Beach is what they prefer.
Enjoy. You wouldn't have gone wrong either way. The Stellaris has been renovated over the past 2 years and the rooms are great. They are also building an adult pool like they did at the Surf Club which might be done by the time you get there. Had a great time at the casino which was the liveliest of all we visited this year.