So I have the YSL envelope bag in a size medium, black on black hardware (TBH, it's been a while and I don't remember the size, it looks like a small size to me but I recall my SA telling me last week that the envelope bag sizing runs small and he said I actually got the medium lol) and I'm actually planning on getting the college bag in a size small, white with gold hardware.
I've had the envelope bag since 2017. It is one of my most worn bags and it has held up beautifully!
The treated leather is very durable and scratch resistant. It holds it's structure very well, I don't need to stuff it and it stands on it's own.
The only con I see is that, because of the strong structure of the bag, it will not stretch much and does not like to be stretched. Once I start over-stuffing it a little, it becomes difficult to close and the magnet closure won't line up to click shut.
I've gone to look at the college bag 2x, and while I have never owned the bag I like it for a few things:
1) top handle, versatile and useful, with a shoulder strap! Best of both worlds!
2) leather flexibility, it is a softer leather, and therefore, can stretch and give a little if need be (I try to not over-stuff my bags, but sometimes I need that smidge of room lol) Also, it will rest more comfortably on the body. The envelope bag's strong structure can make it stick out a tad bit (but it's really not that bad)
3) chevron stitch, personal preference lol, just been really into that lately!
The college bag does have a relatively softer look, just to keep in mind with your aesthetic.
Hope that helps! Both great bags! Good luck!