It depends on how much stuff you put in it. I have and prefer the PM because even though its smaller than the MM, there's still a lot of room in it- I never can fill it up. Also, the straps on the PM are attached by metal loops and you can fold them all the way down when you digging through it, the straps on the MM are attached by buckle and even though then can bend over so you can get in the bag, they don't fold down completely out of the way. Those kinds of details are important to me.
However, the straps on the PM are wider and can be a little uncomfortable to carry in your hand and the 7inch drop is a little harder to keep on your shoulder.
The PM's loop for the handles look cheap... there's more work to do for a buckle strap-like handle on the MM.
PM has way too thick straps to be used as a shoulder bag and even if they were thin, the size is uncomfortable to be used as a shoulder bag you'll need a longer strap length.