You should also consider how long you intend to keep this phone? The 4S has the A5 processor, whereas the 5 has the newer A6 processor. As things stand today, there may not be too much of a performance gap between the 4S and the 5; but in a couple of years or more, newer iOS's will widen that performance gap - for example iOS 6 runs smooth on the 5 (and 4S for most intents); however has a lag on the iPhone 4 with its A4 processor.
On top of that, at some point in the next 2 years, Apple will surely hold back future software features from its older hardware - whether capable or not (like Siri and iPhone 4).
On top of that, at some point in the next 2 years, Apple will surely hold back future software features from its older hardware - whether capable or not (like Siri and iPhone 4).