Building on Horace's comments...
The first Ren I stayed at after the acquisition was La Defense in Paris. It reminded me more of a Holiday Inn or a Days Inn than a Marriott. Nothing really wrong with the place...just totally undistinguished IMHO.
On the other hand, as has already been alluded to, the Chancery Court in London positively blows away most Marriotts.
I'd put it this way....overall quality, value, and service tends to be comparable, but more variations at Rens. Truly apples and oranges in many cases....different, but hard to classify one as typically better than the other.
Marriott taking over the brand was the best thing that happened to Ren. Before that, in my experience, most of them were pretty awful.