Trying to make up for lost time, Marriott has wasted none setting up an immediate Status match website between itself and SPG here:
Marriott Silver matches to SPG Preferred Plus
Marriott Gold matches to SPG Gold
Marriott Platinum matches to SPG Platinum
And points can be transferred back and forth at 3 Marriott to 1 SPG.
So transferring 3,000 Marriott will get you 1,000 SPG
Or transferring 1,000 SPG will get you 3,000 Marriott
Since SPG Gold only needs 25 nights and Marriott needs 50 - and by my earn and burn reckoning 1 MR point is worth 2.5 SPG we are getting the thinner end of this wedge, but LOTS more aspirational properties to earn and burn at, so I reckon it's a canny move.