I'm just short of lifetime platinum with Marriott (about 40 and already have plat for 2016). I can just wait it out at 15 nites a year from my CC. This is what I think I will do as I am about 80 paid nights from lifetime Diamond with Hilton. Looks like I may stop my constant travel for work sometime next year and I think this is the best strategy to reach lifetime top tier with both.
If I make lifetime with Hilton before I cut back on business travel, I'll switch back and finish with Marriott.