I've stayed at both tsjesq and both are absolutely up to brand standards.
The Marriott is right in the centre of Brussels opposite the old Bourse and a quick walk from Grande Place with all the tourist attractions and food/beer options. It's Exec lounge is on the ground floor and open 7 days per week freely serving hot and cold small bites, Beer and wine. If you want a quiet room be sure to ask as the hotel is right in tourist central so revellers can get rowdy especially at weekends, but rooms at the rear are blissfully quiet.
The Renaissance is in the district of Ixelles, about a half hour walk from Brussels centre but virtually next door to the European Parliament. There's a nearby square which is very busy when parliament is sitting but when it's not the area is very quiet. The Ren Exec lounge is on the first floor (US 2nd) and like the Marriott freely serves hot and cold small bites, beer and wine, but unlike the Marriott the Exec lounge at the Ren closes for the weekend.
When I have business with the Parliament I tend to stay at the Renaissance, at weekends and leisure I choose the Marriott. I think the Renaissance rooms are slightly better as is the breakfast there, if the Ren was where the Marriott is and kept its lounge open 7 days a week I would always stay at the Renaissance.
Short answer, weekend or leisure trip, Marriott, visiting EU Parliament, the Renaissance. Mostly the Ren is the cheaper of the two and sometimes the Marriott can be very costly, like €100 more than the Ren, and then I'd choose the Ren, weekend or not, leisure or not...