in my experience the Marriott "guarantee" has been more or less equivalent in practice to the combination of Hilton's soft benefit and asking nicely at the front desk.
I have found the Marriott guarantee to be very reliable, with the exception of one Florida Courtyard that decided to call themself a "resort." (They had a palm tree and a pool.)
I rarely ask for it beyond 2:00 PM unless I'm flying a red-eye from California home to the East Coast. That's something I've done once in the last 3 years and try very hard to avoid. So a 4 PM checkout probably isn't worth chasing Marriott status for. It sounds from the helpful comments here like Hilton Diamond is pretty good for most of the things I care about, but not noticeably better than Marriott. I'll continue using Marriott where it makes sense, but am likely to get an Aspire card. I can easily use the airline fee credit and probably use the other benefits aside from Diamond, so it probably will pay for itself.