Not sure how true this is, in terms of the science behind it, but the iPhone 4 felt a tad bit slower than the iPhone 3GS, for things like loading web pages and such especially on 3G network.
I wonder, wouldn't the best iPhone upgrade be base on having good enough graphical/computational processing power to flawlessly push the display, strong enough network capability to effectively minimize network bottlenecking to the point where it is barely perceivable, followed by a good amount of RAM (preferably of a speedier variety and as much as possible without becoming a power hog) to ensure larger/more application/applications smooth execution?
Won't a combination of these things ensure greater longevity in terms of future IOS upgrades compared to say, a specific attachment of an S or not to it's name.
If that were the case, the iPhone 5 would probably qualify as worthwhile since it has been significantly upgraded in these quantifiable areas.
Not to mention improvements in loudspeaker clarity, call quality, low light photo shooting, stability improvements/sound reproduction in video taking, a lighter/slimmer new form factor, improved battery life and finally an all new immersive, color accurate, 4 inch display that works well both indoors and out.
P.S. Since the 3G, I have had the distinct privilege of upgrading every single year to the latest. The 5 is perhaps the most tangible, well executed, comprehensive upgrade, cumulated in a single year. Sadly marginalized by the longevity of the excellent iPhone 4S, lifted to impartiality by IOS 6.