Since I don't have a store close by, I ordered both sizes and had them shipped to me so I could see them side by side IRL. At first I thought the GM was too big so I sent it back. After a couple of days of looking at the PM, I realized that it was too small. Most of my other bags were PM size, and I had really wanted a bigger bag this time. I couldn't get the GM out of my mind. So, I ended up sending the PM back and repurchasing the GM back (luckily, the day before the price went up).
Keep the GM! It's a better value. It's hard to come across an LV bag of that size with as much detailing on it for that price. Plus, it's more versatile.