Anecdotally speaking, pre-rewards program merger, I found that comparable hotel reviews (at least business/city hotels where I usually stay) on legacy SPG.com averaged ~0.5 star lower than on legacy Marriott.com. After the merger Starriott imported the scores as-is.
I don't think that Marriott scores were "inflated" or SPG scores suppressed, just that each community had differing consensus baseline numbers. Definitely be aware of this and don't simply compare scores between these properties.