Both brands are great as far as lip formulas go. What it might boil down to is the finding the shade that suits your style and tastes.
Going for more lightweight - Dior
Going for bold out there - YSL
Another poster mentioned NARS which is also one of my top favorite lipstick brands (if not #1 in lip products) as well.
Have fun choosing your lipstick!
Choosing between and Dior and YSL is a tough one! Both are amazing brands that offer high performance products. Are you looking for a lipstick that is more sheer or opaque?
Dior's Addict Lipsticks are a combination of a gloss and lipstick in one and tend to be more sheer in color compared to YSL's Rouge Volupte and Rouge Pur Couture which are more opaque in color with just one swipe.
Hope this helps,