It really does depend on the meta.
both are great in their own way. They are stalls. I feel like sinister has fewer fast counters and can just take forever without one.
Yeah without a counter, Sinister is just not even worth doing. And he probably does have less counters
Jean. Sinister can be done by a lot of champs who don't rely on raw damage even dot champs work if you safely play. I have used swomen and spam many times to defeat him who are not his direct counters.
Now jean is stupid easy if you have reverse control immune. But the unpredictable special make you block her specials most of the time just to be safe.
sinister can be an absolute pain if he degens you or heals all the damage thrown at him. Jean on the other hand doesn’t really have any dangerous mechanics except for reversed controls
definetly sinister