You can't get 4.3 and not Knox - Knox is part of 4.3.
The question is why you want to root? If you're thinking that will allow you to get 4.3 without Knox (or get 4.3, then delete Knox), it won't.
Unless there's some specific app you want to run that requires root, you don't gain anything by rooting the phone. (What you lose is any and all warranties and insurance coverage.)