Why not call Hilton and see if they will match your Marriott Plat to HH Diamond and enjoy more perks? That would secure you to gold through next year for sure. I'm sure they have a nights Diamond challenge along with a stays one. IMHO HHonors Diamond/Gold gets you better treatment than Marriott does. I'm an event planner so I have Marriott Platinum status, but I much prefer SPG Plat and Hyatt Diamond to it. I find that Marriott treats its elites the poorest from my travel experience. I have been top tier in all 4 programs, but if I were told I could only be top tier in 1 for the rest of my life, it would be SPG. I rate Marriott 4th of the 4. You have to beg for things like upgrades, breakfast on weekends, and late checkouts.

I honestly think with your travel patterns that you could keep Hilton Diamond -- 28 stays or 50 nights I believe is criteria. I think they do a nice job treating their elites like elites.