Sorry for such a long post.
I have been debating whether or not to get the PS1 as well for years. And finally decided to buy one. I was there thinking for about half an hour between large and XL, with the different color choices!
I must say, it was hard.. But I ended up with the Large Midnight Blue! The color is amazing, quality is just beautiful, and the way it's a bit more angular makes it look like a men's messenger.
The only drawback would have to be the strap length on the Large. It isn't as long as the XL, and I didn't really like that, but the XL in my opinion was a bit too large (Although I do use the Keepall 45 for everyday). For reference, I'm 5'9" and average. The size of the large is a good proportion to my body, except the length, but I am okay with it since it will eventually wear and sag a little. And the $300 price jump was another reason why I was a bit hesitant.
Good luck to you! I believe it's a great investment piece. Classic and will last you for a while.
Oops, just saw you made a new post!
1) I'm leaning towards the Large PS1. Medium was definitely out of the question, and I was worried the XL would be too big for everyday use. I usually carry my macbook pro 13-inch, laptop charger, water bottle, pencil case, wallet, and other small misc items. Would all of these fit inside a Large PS1?
My sales associate told me that the 13" is meant to fit perfectly into the large and an ipad in the front pocket of it.
2) Is the PS1 sturdy? I keep seeing posts about screws falling off/loosening and seeing as I plan to use this as an everyday school bag, I'm worried it'll fall apart while I carry my school stuff.
By comparing the large and XL together today IRL, I noticed that the XL has a strap that wraps around the metal where it hooks onto the bag and looks a lot sturdier than the large. The large has a somewhat skinny metal hook thing that seems like it would snap if the bag was too heavy. Kind of sketch. I'll have to see how that holds with my ipad and other miscellaneous items.
3) How long is the strap? I want to carry it on my shoulder as well as crossbody but I'm not sure if the strap is too short. I'm 5'10".
I kind of talk about the strap being too short, it does cross body, but ends up being above my butt area, so like in the middle of your torso-ish. I kind of have a long torso, that might be my reasoning.
4) I know this is more of a personal taste kind of question, but which colour would you pick: Black or military?
I saw both, including the Midnight Blue, I seriously have to say, they are all equally amazing. I went with blue because I have plenty of black bags, and for military, I wasn't bold enough yet to reach out of my dark/universal/practical color.
Once again, this is an extremely long post. I practically wrote a short essay. I tend to describe a bit too much. Hopefully this was helpful?
It's too bad there's no Proenza Schouler store in Canada... (unless there is... someone please correct me if I'm wrong!) I guess I'll wait til my next trip to NYC 
guess I could order online...
