I've stayed at both. The Marriott is a perfunctory FS. The Ren is a standard Ren, if there can be such a thing. The last few times I've been in your situation, I've gone with whichever one was less expensive, which as usually been the Ren. Neither is a bad choice.
Although you indicated you needed to be near the airport, the Marriott West (I may be wrong about the exact name) is usually less expensive than the airport hotels and is nicer than either of the airport hotels.