Yeah I might hold off a few days to hear what people have to say about it on the 4S. I'll upgrade my iPad mini 1 and play around with iOS 8 while I'm waiting
Every iPhone gets cut features past the initial version of iOS it comes with. Some things minor others not. The 5S will be missing something like every iPhone before it, probably most will consider whatever that is to be insignificant.
The 5S will get everything that has been seen so far. The iPhone 6's hardware will be what the 6 gets over the 5S. So far it's reasonable to think of the following as pluses over the 5s.