Have stayed many times at the Marriott and always been pleased with the hotel, service, and location. The subway (U-bahn) stop is quite close (<5 minute walk); 10 minutes on the train and you're downtown.

Can't offer a direct comparison as I've never stayed at the Ren, but both hotels are fairly near to one another. Since you can use the W14 at the Ren, I'll concur with the advice of the previous posts. Gute Reise!