I have the Schwalbe Marathon Plus tires in the 1.35 size on my Dahon with 20" (406) wheels.
Schwalbe also makes the tire in 1.75 again for a 20" (406) wheel.
I'm not too familiar with Downtube, but I believe you use a 406" wheel right? Not the larger 451?
It is up to you what the extra puncture protection is worth...
Certain the tires cost a fair bit on their own as does any quality tire.
The tires are signficantly heavier than other Schwalbe tires with less puncture resistance.
That said, for me, not having to sit at the side of the road (in the rain or snow) with a tire lever or having to phone my gf to come fetch me... yes that is worth the trade-offs for daily riding. These tires are darn near bullet proof... I have not had a single puncture since fitting them. While I wouldn't want to tempt you into purposefully aiming for patches of glass on the road, I would feel confident riding over glass whereas other tires would be swiss cheese. If you want to do a little weight balancing, you can afford (from a puncture perspective) to get lighter weight tubes (Schwalbe make a set e.g. SV-6A).
Other tires I have personally ridden:
Schwalbe Marathon Slick
Schwalbe Big Apple (on my gf's bike only...)
I got flats on both the Slick & Stelvio and the Big Apples haven't been any where near as far as the other tires have - but the miles they have done, they did not get a flat either. Some people recommend Big Apples as an alternative to the Marathon Plus, but they are a bigger/fatter ride so it depends what you want. If your objective is to banish punctures forever, you will be very pleased with the Marathon Plus. You can feel the difference right away even just picking up the tire and comparing to another - much more rigid/thicker tire. If you get the 1.35 you don't even have to look like you are riding on tractor tires.
(I'm a skinny tire snob...)