For a few days I have iOS 5 beta 3 on both iPhone 4 and iPhone 3GS, both terminals working without jailbreak. Although iOS 5 beta 3 seems to be the fastest beta version so far, compared to iOS 4.3.3 there are big differences and not in a good way. Unfortunately, things "move" slower in iOS 5 beta than in iOS 4.3.3 and I expect the performance to remain the same in the final version. Since the release of iOS 5 beta 1 it was said that The iPhone 3GS has similar, if not identical, performance to the iPhone 4. Well, things remain the same even now after the release of the third beta version.
iPhone 3GS is a little "slower" than iPhone 4 in terms of opening applications, but the difference is of the order of hundredths of a second. Almost every application opens equally quickly on both terminals, except (for me) App Store and Messages, but the rest work almost as well. Regarding the loading of web pages, there are big differences, but everything depends on the site, sometimes the iPhone 4 loads the pages faster, other times the iPhone 3GS comes out victorious. The iPhone 4 has a faster processor, but it has a screen with double the resolution compared to the iPhone 3GS, and that's probably why the performance differences are minor between the 2 terminals.
If things stay the same until the end, then the iPhone 3GS and iPhone 4 should work about as well on iOS 5.
