The difference is expectations. Marathon Supremes are under the Marathon line and are marketed as commuting / touring tires. Continental GP5000s are no doubt far more fragile, but they are marketed as race tires. Nobody expects those to be durable.
pretty much a moot point and they are no longer in production.
As I've noted over the years, my Supremes very much exceeded my expectations, being pretty damn durable while riding loaded through all kinds of countries, plus having a nice ride-- but very much with the caveat that I've been always very aware of the thin sidewalls and ride accordingly around sharp gravel, curbs etc.
For the majority of riding that I do, I'd be happy with a similar type of tire. This summer I found myself on some rather rocky gravel roads with the 1.6in Supremes, and really took it easy when the stones were bigger and it seemed logical that the risk of damaging a sidewall was getting more, so I just took it really easy and carefully over those sections.