It's definitely a personal choice but, traveling by myself, I would stay at the Ren over the FS Marriott.
A few other points in the plus category for Ren :
* Exec lounge on top floor so can walk there without taking elevator. Unless it has changed, Marriott has their exec lounge on lobby floor so have to take elevator down/up each time.
* Have people mentioned prices? :-) Can almost always get a room at a good price.
* Room is small but, surprisingly, cozy for me. I request a runway view and almost always get it.
* Fairly quick walk into Sipson to a great pub : King William IV
Now, if I were staying with my family, I would probably grab a room at the FS Marriott as they are definitely bigger.