Now that we can match status between Marriott Rewards and SPG, I've been able to try out both RewardsPlus, the partnership between Marriott and United and the SPG/Delta Crossover Rewards.
First a little background
Marriott Platinum elites with a United Mileage Plus account can link accounts and get automatic Silver status with UA.
SPG Platinum elites with a Delta Sky Miles account can link accounts and get some modest benefits when flying DL.
The UA Silver status is self-explanatory.
The SPG/Delta partnership needs a bit more description. It does not grant even the lowest level stats with Delta (Silver Medallion). It does give priority check-in, priority boarding (zone 1, right after FC and Sky Priority), a chance at upgrades (although last in the queue), first checked bag free, and the ability to earn 1 Starpoint per dollar spent on Delta flights (excluding taxes and fees).
On my recent DL flights, I was last in the upgrade queue both directions between CMH and ATL. While I did not get a FC upgrade in either case, I was upgraded to Comfort+ on my flight home. Being last on the list when flying into or out of a hub like ATL, it's extremely rare to be upgrade, so I was surprised and pleased to get the last available premium economy seat even if the flight was just over an hour long.
Sometime in the next 4-6 weeks, I should see about 200 Starpoints added to my account which will bump my grand (lifetime) total up to about 200. These points aren't something that mean much to me (yet), but if I can earn Delta Sky Miles and SPG Starpoints for the same transaction, I'll gladly collect both. It feels a little like getting something extra for free.
In my opinion, getting automatic Silver status with UA is a better perk. Upgrades are more common and mileage earning on UA is better at the the Silver level. Admittedly, we don't earn any MR points for money spent on UA flights
It's reasonable to guess that one of these two partnerships will eventually go away. Given my experience so far, I'm rooting for the UA partnership to stay. Neither one offers a lot, but of the two, it provides slightly better benefits.