I am a very frequent traveler to Mexico City.
I have stayed at both properties. I would stay at the Sheraton. It is very close. One and a half blocks away. Tha walk is not bad except for crossing Reforma.
The Galeria Plaza was going downhill when it was a Westin. I have walked by it a few times this past year and it was looking "scrungy".
For the points I would pick the Sheraton.