As of September 2016 Marriott purchased SPG. If you have status with any of the following (high enough) then you have status at them all!
Marriott Platinum or SPG Platinum gives you the other. http://Members.marriott.com
Marriott Platinum Status gives you United Silver – https://mrrewardsplus.com
United Mileage Plus Gold or Higher Gives you Marriott Gold – https://secure.unitedmileageplus.com/reg/rewards_plus/index.htm
SPG Platinum gives you Delta Silver –https://www.starwoodhotels.com/preferredguest/account/benefits/delta/confirm.html
Delta Diamond or Platinum Medallion gives you SPG status – https://www.delta.com/profile/spgLanding.action
Basically it is a very good day for domestic travel.