In most cities (not all) the Ren is more modern/stylish and has a more luxurious feel than the traditional FS Marriott (especially domestic properties).
What does "FS Marriott" mean? I've seen another post here about bathroom amenities at FS/Marriott.
On the Marriott forum FS means full service properties like Marriott & Renaissance compared to other brands such as SHS, RI, FI which are not full-service. FS takes less time to type than full service