I think there's basic difference between the M/L Classic and the Mini Rectangular that's being overlooked.
The M/L is a shoulder bag, with a strap too short for most women to wear crossbody. The Mini can be worn long as a shoulder bag or, most likely, crossbody. As for capacity, the Mini holds nearly as much as the M/L. Compared with a WOC, the Mini holds a great deal more -- including room for a car key, if that's important to you.
If it comes down to Caviar vs. Lambskin, I've had both and have a distinct preference for luxurious Lambskin. In fact, recently I sold my 2017 Black Caviar Mini and replaced it with a pristine pre-loved 2021 SO Black Lambskin Mini. In Caviar, in my opinion, it's only a casual bag. In Lambskin, it goes day to night with ease and elegance. It's equally wearable with jeans and at a cocktail party. Best of all, with a Mini you're hands free.
I should add that I had a Black Lambskin Classic in the hard to find small size that I bought new eons ago and used for many years. At first, it was my every day work bag. Later, it became my going out for dinner bag. All the concern some women express about Lambskin being too delicate has not been true in my experience. My Lambskin Classic looked nearly new when I sold it a few years ago and fetched top dollar.
Finally, I'm such a fan of the Rectangular Mini that I've just added a second one -- a new Metallic Pewter with silver hardware -- to cover all the seasons.
Good luck with your decision!
Millstream