We just got back from Amsterdam and stayed at the Marriott 4 nights on points and then moved to the Renaissance for a 1 night paid stay. We received a Platinum upgrade to an Executive Level room free at the Marriott. At the Renaissance we were not upgraded to an Executive Level room but had access to the Executive Lounge (again no upcharge).

We were at the Marriott over a weekend and even though there is no food service until Sunday night the lounge was accessable with our key so that we could get drinks (non alcoholic) and use the computers. The breakfasts were great and there were hard boiled eggs, meats, cheeses, assorted breads and croissants, cereal, fruit,yogurt, etc. The evening offerings were great, too. There were two hot dishes, sandwiches and wraps, beverages (wine, beer, etc.), several salads and great desserts.

We didn't get to fully experience the Renaissance Exec Lounge for the evening, but breakfasts were similar to the Marriott's. We had dinner reservations that night and only saw the cold hor 'deurves and one hot dish (more could have come out but we weren't there if they did). They had several types of Port in addition to the wine, beer, etc. We came back after the dessert so really didn't get to see what the Renaissance offered. The Renaissance Exec Lounge is huge (only one computer) and the people who staffed it were very helpful.

We were in Brussels, too, and were upgraded to Exec Level. On the weekends the guests on Exec Level get to eat breakfast free in the Restaurant since the louge is closed (don't know policy for Plats or Gold who aren't on the Exec Level). The Lounge is available on the weekends for soft drinks, etc, and computers.
During the week the evening offerings there were outstanding(assorted small individual salads each night, rack of lamb and pasta one night, pork tenderloin and pasta another night, and fillet of beef and pasta the third night). They also have two computers in the Lounge -one with a US style keyboard and one with a European style keyboard.

I would definitely stay at the Marriotts again in Brussels and Amsterdam and would choose the Marriott over the Renaissance in Amsterdam due to location and atmosphere.