Yay, a newcomer to Goyard Anyways, this is a difficult question! What exactly are you looking for? I think the main differences to take into account between the Fidji and the St. Louis PM are:
1. The Fidji has one strap, the St. Louis has two.
2. The Fidji can be zippered shut, the St. Louis cannot- it remains open.
3. The Fidji is more of a slouchy hobo shape, whereas the St. Louis is more of a tote.
There are, of course, more differences and I'm sure you were already aware of these excessively simple differences! I find that it's helpful to put everything down, though
As an owner of both a Fidji and a St. Louis PM, I find that I love them both but they have their specific uses. I use the PM as a schoolbag because it is unbelievably durable and fits more, and the Fidji as a hobo for leisure time/hanging out. I also tend to use the Fidji for travel because it is zippered, though I just used the PM as a carry-on for the first time and it did work well (nothing fell out! hehe).
I hope this helps! OH and also, keep in mind that the St. Louis comes in two sizes, the PM and the GM, so if you opt for the GM it will be larger than the Fidji. Best of luck!!!
I agree with annalcg, what are you looking to use the bag for? I have both a St. Louis and a Fidji and love them to pieces! The St. Louis is built better for a school bag because it fits books + notebooks better than the Fidji. A notebook might be a bit awkward in the Fidji. They are both wonderful and sized just right as daily bags for me, and I rotate mine frequently along with my all Balenciagas HTH!