The PM is a decent size and that's what I had. I'm 5'6. Unless you plan to use it for traveling, kids or whatever then the PM should be plenty big enough.
I don't like how the GM's handles flop about. I prefer the way the PM's handles fold over nicely. Plus, the long strap for the GM isn't really big enough for cross body (for me anyway! 5'6" and plus-sized)