I love my 5s. Picked it up on launch day unlocked.
However, I agree that you should wait until September. Might as well see what the 6 has to offer. Unless you are in absolute need of a phone.
Will I upgrade? Probably not. But I am curious to see the changes, and I would likely upgrade to a 6 if I was still on a 4s. When the 6s arrives one year later - I will be all over it, guaranteed. I have just found, between the 4, 4s, 5, and 5s - that the s models are the same, yes, but have all the flaws and bugs worked out with newer tech installed (e.g. battery issues, display issues, better anodizing, cases introduced, etc...)
Wait and see, though. It's a win-win if you wait, as per mentioned above.