Kassandra- like you I own a large black campana bag, and I currently have a medium black veneta.. The differences I find in veneta vs. campana are: 1- The campana has a flat bottom ( the depth of campana is at least 6") and can hold more than a lg. veneta would...2- Campana has an open top, and it opens wide, so that you can easily reach in to retrieve your wallet ..you can see it , you don't have to fish around 3- veneta has 1 shoulder strap which stretches over time with everyday wear 4- Campana has 2 shoulder straps, that yes sometimes, one slides off the shoulder, but if you hook one over the other, they both manage to stay put.. 5- the large campana falls a little lower on the body, I think it is a bigger bag lengthwise. 6- Veneta has a zippered top, which some people need for security, while the campana has an open top with a dog leash style clasp that is optional to hold the side together..
If you buy a large nero veneta , it will basically work the same way your pleated veneta works for you, except I do not think there as much depth in the traditonal venetas as there is in pleated ones.
For me the veneta would be my next bag over the montaigne, because I like an optiion of a shoulder strap, the braiding all around..That is just IMHO