I guess the question depends on whether MSC has updated their match charts with Marriott's new elite levels. If so, then since today's Marriott Plat really matches yesterday's Marriott Gold, one would be matched to MSC Gold. So given that Hilton Diamond is their top level and Marriott Plat is 2 levels below the top level (below Plat Premier and Plat Premier with Ambassador), I'd suggest going with the Hilton Diamond. But the is just guessing on my part as I haven't seen or heard of recent matches.