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Aside from screen size how much can they really change? We're otherwise dealing with nuances now.

One thing I hear often is battery life and that is really the only meaningful (to me) unknown about the next model. However, early reports don't indicate a big increase.
 
The 5s will be a big upgrade coming from a 4S, however the 6 will be even better. It's only a month away, it's your call.

I would not recommend anyone to buy an iPhone until next month after they launch the iPhone 6.
 
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