Got the MM then returned it because I thought it would be too big. Fast forward 2 years and I bought the PM. It's ok but realizing I would have liked the MM better. I'm still happy enough with my PM though.
I love my PM & I am typically a "big bag" kind of girl (artsy mm, neverfull mm, speedy 30) considering I am only 5'0 and petite. I went in with my mind set on the MM but it was too big for me.. I felt that the PM held a ton and was slouchy but still looked "neat" compared to the MM looking super casual on me. You can't go wrong with either. It's a personal choice.
I have the Delightful PM and I think she's perfect. I love big bags, but as a mom things can get lost at the bottom. I feel with the PM it's just big enough for everything, but I can still find my cellphone and sunglasses. And she's slouchy enough without being messy.