Hello! I JUST got the GM on Monday! I have two other Neverfulls, both MM size, and in my opinion, the MM is the perfect size work bag (for carrying files, a slim computer, and daily essentials). Everything fits, but it's not so huge that things roll around in there. It keeps everything neat and tidy and there's still room to throw in a scarf or something on top.
I got the GM because I'm a mom, and in addition to all of the aforementioned stuff, I often have to carry jackets, Taekwon Do uniforms, water bottles, snacks, random and assorted art projects, and God knows what else. LOL. The GM is *perfect* for this. I was able to carry most things in my MM, but it was really stuffed and often things were spilling out the top. The GM holds everything comfortably. The downside of the GM, of course, is that it is a black hole. An awesome, glorious, cavernous black hole!
Let me tell you something; this morning we couldn't find my daughter's jacket. Looked everywhere for it. A few hours later, I found it in my bag. Just, like, bunched up and lost in a corner of my Neverfull GM. A WHOLE JACKET! This bag is large, okay. Things SWIM in it. In the MM, things stay nice and tidy and upright; in the GM it's a floppy, chaotic free-for-all (but I love it).
Hope that helps. For the purpose you described, I recommend keeping your MM.