Lately I've been finding the cash rate which earns stay credits and MR points is often a much better deal than using points for individual nights. To figure it out, look for the lowest rate you can get, AAA or Senior, and compare it to Marriott's price for buying points.
Example: if they want 20,000 points for a night you could buy them or $250. If the best cash rate is close to that, or less, I'd pay cash and earn points/stays. If the best cash rate is $350, reward points would be a better choice if available.
As mentioned, however, depending on how many points you've already got, the best possible deal is using them for air+hotel trips...5 or 7 nights.
Brian