If you are looking to get status with Marriott for Platinum/Gold perks during future stays, then the RI may be OK. The only Platinum perk that would be of significance at a non-full service property is the 48 hour reservation guarantee. If you plan on staying a full service properties in the future, the Platinum arrival gift, lounge access and possible upgrades are more tangible perks.

If you are looking to earn points for free stays, then the RI is a poor choice because you only earn 5 pts/dollar versus 10 pts/dollar at other Marriott properties.

I don't know about Staybridge enough to compare the two, but if your assignment is 6 months, why not balance your stays at both properties and earn elite status with both?