If you're going for a timeless look
regardless of what you want to use it for and how much stuff you want to fit into a bag, then I'd go with YSL.
- Personally I'd go for leather over nylon for a classic bag (though, Prada nylon is timeless to the house itself).
- The chevron stitching is very very timeless and classic, as seen with many other luxury brands most noticeably with Chanel but also
Gucci, and Givenchy.
- The YSL logo has been there right front the beginning when Cassandra drew the logo for Mr. Saint Laurent and it has basically not changed over the years (unlike Dior for example). And the way the logo is placed is pretty much the same for all YSL bags, usually right in the center, used as the bag closure. (Not saying Prada's logo is not timeless, YSL's logo is
very integrated in the look and design of the bag, For the Prada re-issue, it's small and not as noticeable.)
On a side note, just be aware of what you want to use your bag for and what you carry on a daily basis.
The Prada can definitely fit more (I don't have the nylon version, but I have the leather version).
I currently have 3 YSL bags and have owned 3 others in the past. From my experience YSL bags with grained leather (not talking about the loulou and puffer bags) (1) tend to run small and (2) the leather, while very durable and longwearing, is quite stiff, will not stretch over time, so there is generally less room for items because the leather will not give way (or will only stretch just a little bit).
Just some food for thought!

They are pretty different bags so I can see how it can be a hard decision. Good luck!