Got an email from SPG that the three above hotel programs would be combining into 1 come this August.
In August all three brands will be combined into one with a conversion on points and status:
In a one-time conversion in August, Starpoints® will automatically become points and your Starpoints balance will be multiplied by three.
https://members.marriott.com/spg/
Not sure what this means to the many that hold AMEX cards linked to SPG. It might be worthwhile to hold onto the points or move them out prior if you get a better conversion rate.
Could the hotel/point fanatics out there chip in with their 2c? I am not that knowledgeable on hotel points.
SPG to Marriott has always been 1:3 so that's no different to now.
Plenty of articles (local or US based) on the subject.