I'd choose by location. The Renn isn't really "downtown" - defining downtown as "I can walk to my client site which is downtown". But if you're going to the Market Center/Tradr Center, the Renn is a better choice.

I've stayed at the Marriott many times, and it's fine - better since the reflag from SPG and investments. The Renn was fine for my 1 stay, but that was during the NBA playoffs with nothing downtown available and I had to cab it every day.