FYI I'm also both, but my work means over 150 nights a year so I don't have to choose. However, if I were you personally I'd stick with Hilton and accept the soft landing from Marriott Platinum to Gold, which gives mostly the same perks anyway for next year (Lounge, Breakfast, potential upgrade) and then reassess your status next year when you start to travel once more.

My reasoning is that Hilton's diamond welcome points plus airmiles, if you choose them, make your stay pattern much more valuable at Hilton (with the 75 stays for Mariott you'd earn at least 1 night via the welcome 500points, but probably 2 at Hilton's nicer hotels (Cat 7 and above) - plus plenty of airmiles).

You should check out the Hilton forum for the current diamond targeted promotion with one free night cert per ten stays, so three free nights for your stays this year too which is going to be better than the maximum 50k points you get from Marriott for 25 stays via their Q4 visa promo. I think the Diamond perks at Hilton, especially the diamond guarantee for using that huge pile of points you have beats the availability issue around having a choice of hotels in a city where you have status.

Next year if you miss Marriott then its only 75 nights to requalify The roll-over benefit mentioned above probably won't benefit you as I suspect that getting say 15 nights this year would be lost, as they won't be above your tier status level (75 nights) so would just disappear. You may also find that Marriott keeps you at Platinum for a further year, though I think that is unlikely as your status match and lack of stays wouldn't show them much in the way of "loyalty" to be rewarded with an unofficial extension!