Hmm. As I said in the Glasgow launch thread, it was always my plan to buy the next iPhone for both myself and my wife, and sell my current iPhone 4 to fund an iPad. This was due to happen in March next year, and I'm looking forward to it.
However, after watching this video, my original thoughts have been proven. In general UI navigation and use, the iPhone 4 is only marginally slower than the 4S. The difference in a comparison of A4 vs A5 isn't going to be accurately shown until the likes of Infinity Blade 2 launches, and we can see how good an app written with the A5 in mind, looks versus the A4 enabled version.
iOS 5 has made the iPhone 4 far smoother. The UI kit is definitely smoother than 4.3.5, it's fantastic, but as I said, the UI kit and navigation of the OS is not where the A4 vs A5 will be tested. I just don't know now. For someone coming from a 3G or 3GS, the iPhone 4S will be blow away.
If you're not interested in Siri (and to be honest, it's not a feature that sells the 4S to me personally), then the 4S could be a bit of a let down to some (not all) upgrading from an iPhone 4. For me, the A5 chip is more future proof than the A4 and as a massive iOS gamer, I'm looking forward to some of the more advanced titles that will come. However, I don't think we'll see all these super advanced, A5 enabled titles, right away. Developers are still tossing out amazing games on the iPhone 4 (ShadowGun, as shallow as it is, looks phenomenal).
Come March, it'll be seven months until the next iPhone (assuming Apple have moved the iPhone launch to October and merged it with the iPod event). My issue is that I may not have the money come October to splash out on two new iPhones, so it may be best to pick up the very best there is at the time I have the funds, and take it from there.
Right now though, my iPhone 4 will do me just fine.
Edit: I may even jailbreak iOS 5 when a tethered option is available for the iPhone 4, and install FakeClockUp to make the smoother animations go that little bit quicker.