RC doesn't do much for MAR/SPG elites, as it's a bit of its own program (even though part of Marriott). You'll basically get free wifi. You have the possibility of an upgrade, but as with all upgrades that's dependent upon how full the property is, whether others have booked/paid for upgraded rooms/suites, and how many Plats and other Golds have checked in before you. You will not get a free breakfast. If you want club lounge access, you'll need to pay for it.
Cheers.