Comparing this with the Ultra slim eyeliner. I don't really see a difference when it comes to line thickness, you can draw thin lines with the Skinny fit slim pencil just fine. 

On the pro side for S eyeliner:

 It's got a softer formula compared to U eyeliner, so it's easier to draw without too much pressure of the hand

It's less likely to break considering the thicker pencil 

Cons: 

It's not as smudgeproof as the U eyeliner witch you can tell by the pictures where I compared them after smudging with my finger. 

Perhaps a tiny bit lesser precision comparing to U eyeliner