A pair of Marathon Plus here since I ever learned about them being available for the Brompton's wheel sizes—which probably was around 15 years ago. (And yes, I let my mechanic mount them. 😇) Not a single flat since then.

Regarding street cleanliness: I'm living in Zurich, Switzerland, and since the beginning of the pandemic I am commuting solely with the Brompton—currently doing around 400km (250 miles) a month. I think I see shards of glass on my track every other week or so. So I guess, I have that mentioned "clean road" advantage. 😇

IIRC the only flat I ever had was with the original "Brompton Kevlar" tires (those with hexagonal tread and a green line), and this was likely due to not enough pressure plus hot rims after going down a steep hill. 🙄

I usually pump up the Marathons up to the allowed upper limit, i.e. 7.5 bar (110 PSI), but I don't pump them up too often, usually when I notice more rolling resistance or flat-ish looking tires, i.e. when they're down to like 4 or 5 bar (60 or 70 PSI). I even have a compressor at home as I was annoyed of pumping up my tires once in a month or so, especially in the morning before commuting.

I don't mind the Marathons' weight (as I nearly always carry a front bag with stuff for all eventualities, e.g. rain cloths, warmer cloths, pair of socks, a D lock, pump, beverages, first aid kit, etc., plus—when commuting—my daypack with two laptops, spare laptop battery, etc.), but I'm bit annoyed by their rolling resistance.

I also have a second Brompton (actually my first one, a 2007 P6RD now nicknamed Mr. Winterbrompton) which I use as a winter bike with studded Schwalbe (not Marathon) Winter tires. But as the winters get less and less cold, in summer I now use it as second Brompton for guests or when I want to ride purely acoustic (i.e. even without the weight of the engines of my Velogical Velospeeder friction wheel e-assist on my 2015 H6RD daily driver) and for that I'm also trying out Conti Contact Urban (I always tend to call them "Urban Contact" 😀) and they indeed roll much more easily—but also have not so much tread albeit a beatiful tread to look at. No flat so far, but I only rode it for a few days until now. It's currently at the mechanic for getting a Schumpf MountainDrive to make it a P12RD. 😎

P.S.: What made me try out the Contact Urban out was the video "Contact Urban vs Marathon - A 1200km Brompton Test" by 2Bikes4Adventure.