That's assuming the ZSS plays perfectly however, which isn't really a thing in Smash 4 yet, keep in mind everyone it's difficult to definitively say since people are STILL not good enough at the game to determine who the best character is yet.
I'd say Sheik is the best since she's the best at avoiding damage, she's safe on most of her spacing moves, her neutral game is just silly really, plus she has needles, she's literally the best defensive character in the game, that much can't really be argued, ZSS can put up a good wall too but it doesn't compare to Sheik's absurd rubbish which makes her a pain, I mean
The problem that ZSS has is that her combo start ups are unsafe, her grab for example puts her at a disadvantage, her throws and combos are godly, better than Sheik's, but her grab can leave her at risk, unlike most of the moves that Sheik players use.
As I said the question is very hard to answer since believe it or not things are still early days, but I'm leaning more towards Sheik just because of her safety, what makes her so strong is her neutral stance.
in terms of who i prefer theyre both overused like crazy im tired of watching them
I decided that I wanted to main Sheik before the game even came out because Sheik is simply cool in character and design, ninjas are cool, I also player her a lot in Melee, and I was happy that she was really good in 4, but then I got bored with her and swapped since everyone used her, and I was just another boring Sheik main, not trying very hard to do anything cool.
It's the same case with ZSS, just not as much, I wish every character was as good as ZSS.