I was in the boutique a couple of days ago to try on both of these bags, and came away much preferring the Disco (it's next on my wish list!). It seems to me that the Marmont photographs better than the Disco, but is somehow a little lacklustre for me irl? The disco is plainer but that makes it more versatile.
More importantly, I think the Marmont's shape is slightly odd on me, it's width and height somehow felt a little awkward overall, like it's slight disproportionate. I'm 5"3 and about 120lbs) We're the same height and both bags can work as both a shoulder and cross-body.
But the Disco is more pliable and also lighter than the Marmont because it doesn't have the chain. The Marmont is a little dressier than the Disco though, so if you like your purse to be the 'centerpiece' of your outfit, I would go with the Marmont. Otherwise, i think the Disco wins in terms of functionality and ease of use.