They are both very nice hotels.

The Sheraton is a 5 minute walk from Zocalo Square. Safe area for daytime walking.

The Marriot is in Polanco. Polanco is a more trendy, swanky area of MEX. I usually stay next door at the IC Presidente when I go for business because the area is nice and more relaxing.

To get more of a taste for the city and the sites I'd opt for the Sheraton.