SPG matches Marriott, but not vice versa. At one point I was Diamond at Hilton, Platinum at SPG and Silver in Marriott. Marriott would not even consider a match.

Now for the good news. There seems to be a program within MR call "Fast Track". It allows a one level bump based on reduced nights, one time for elite members of other programs. I had to provide statements from SPG and got the bump from Silver to Gold after 25 nights. It does not uniformly reduce the number required however. The Platinum remains at 75 nights