Jessie Mueller probably has a contract for a year, so she'll be in it until spring '17. Of course she could always extend, I think she was in Beautiful longer? Or she could have negotiated a shorter contract if she has anything else lined up... which is possible but nothing I would have heard anything about. I think it's pretty save to say she'll at the very least be in it until the end of the year, quite possibly longer.
Like others have said, Jenna is only filling in for Kimiko Glenn, so if you really wanna see her, this might be your only chance... if you get to the city at all before she leaves again.
In my book, Waitress is the better show. But like was said before, they are vastly different. One is pretty much a dance show with not too much of a book that's all about... well... Cats. And the other is a more traditional musical in form, but with music by Sara Bairellis (did I spell that right?). It's story about love and friendship with amazing performances all around and I can't get the music out of my head. Definitely one of my favorite shows of last season.
I should add that while I have seen Cats, it was not this production.
