I would say "no" to upgrading from a 4S to a 5. If you had a 4 or lower, then I would say it's a no brainer. But,with a 4S on iOS 6, there really isn't much that the 5 can do which a 4S cannot.
Bigger screen
LTE (that's IF you have coverage in your area, many do not yet).
HD FaceTime (IF the person on the other end has one too).
Take still pictures while recording video.
Now, are those things nice to have? Absolutely, if you are able to take advantage and make use of them. The 4" screen is nice, but, as someone who owns a 4S, I guess I can't exactly miss what I never had to begin with. Sort of similar to the retina display when it first came about. Once I made the jump from 3GS to 4S, there's no way I could've ever gone back. But, up until that point, I couldn't miss a retina display because I'd never had one at that point.
LTE, I assume, would also be nice to have. But in reality, many customers (especially with AT&T) do not have coverage right now. Even so, with the gravitational pull of the shared data plans, and with individual data plans at either 2 or 3 GB per month, it does not take long at all to burn through a monthly allotment of data with LTE. So, as nice as LTE would be to have, given the practicality of data plans nowadays, it's almost too risky for some folks. For now, I'm perfectly fine with AT&T's "4G" network, as it gets me anywhere from 2-6mbps down and about 1-2mbps up, and that suits my needs just fine. Given that I'm still on unlimited data, I have no complaints in that area. Plus with having WiFi at home, work, and on campus, there really isn't much time within my daily routine where I'm not at least near a hotspot.
As for the other 2, I'm not sure how practical taking still pictures while recording video might be. I know that when I'm recording a video, trying to get a still picture would be the last thing on my mind. I'm in "filming" mode at that point, not "snapshot" mode. And HD Facetime will obviously only work if both parties have an HD Facetime enabled device. Granted, it will be a nice bonus for me once I get it, but, again, I can't miss what I never had.
In the end, the 4S gets all the features of iOS 6. So, aside from the A6 processor, the extra RAM (the 5 will load things a few seconds quicker than a 4S), and the aforementioned features, I would not say the difference in daily performance between a 4S and a 5 is worth upgrading.
I personally will wait for the 5S to bow in (probably) the fall, and then I shall upgrade.