St. Laurent gets my vote, I've never cared for the LV logo'd stuff. Too obvious.
I like both. If you aren't anti-monogram like some people are, then the monogram on the LV backpack shouldn't be an issue. I assume you have nothing against monogram print or you wouldn't like the Palm Springs backpack so much. So don't let the naysayers who don't like monogram influence you. Buy what YOU like. If I were to pick my favorite of the two just based on looks, I prefer the LV. But that is largely because I don't like carrying black bags. I don't mind the monogram and I assume that you don't either. But this is not about which bag I prefer or which bag someone else prefers. It is about what YOU prefer. Does one of the bags work better with your wardrobe? Where will you be carrying the bag? If it is to work, the YSL looks more professional but if it is going to be a casual, day-to-day bag, then either bag would do the job well.
That said, have you tried on both backpacks? What is the price of each style? I know the LV is made of canvas and leather. What is the YSL made of? Is it nylon...kind of looks like it? What are the compartments like inside? Does one have better organization? The YSL front pocket looks bigger and more accessible than the front pocket on the YSL. If the YSL is nylon, then it should be durable same as canvas from LV. Do the YSL straps have nice padding like the LV straps...that is a major plus for the LV bag...comfortable to carry because of the padding especially if it gets heavy. The LV also has padding on the upper handle which the YSL does not look like it has any padding there so it would probably be more comfortable to hold the LV by the top handle should you need to do that. The top handle on the LV also is much wider so you can easily fit your entire hand in the strap with no rubbing. Do you have a preference for gold or silver hardware? That is another huge difference between the two bags.