I’ve had both, a vintage montsouris mm which I actually ended up selling and I now have Palm Springs PM. They are roughly the same size in terms of what you can fit inside.
The montsouris is more feminine looking especially with the straps but I didn’t like getting into it with the buckle and string method. I found it was a hassle which is why i sold it.
I originally bought Palm Springs mini and really liked that you could wear it cross body but it was just too small to be useful which is why I decided on the PM size. I think the backpack is very comfortable and I prefer the zipper opening (though it is still tough on my new model).
I would definitely recommend to visit a store to look at montsouris and see if you prefer that opening. I know many stores don’t have Palm Springs in stock but the PMs are set up functionally like typical backpacks. The new montsouris is also very similar to the vintage ones.
Hope that helps - either way I think the backpacks with LV are very popular and have held their value. Even with the backpack trend lessons, people really like backpacks for travel purposes.