OS upgrades that a lot of Android users don't get. If I want to get an up to date browser on my Nexus One, I have to use Firefox... as Google said they no longer support the phone.
iOS users are in the same boat, but Apple has been providing much more support to older devices.
Weird, my 3GS seems no different on iOS 5 vs iOS 4. At worst, it's nothing like the iOS 4 performance on the iPhone 3G. This video apparently shows the 3GS being faster than iPhone 4 (believable, actually, if you consider the quadrupling of pixels on the iPhone 4 from the 3GS): http://techuncover.com/blog/tag/iphone-3gs-is-faster-on-ios-...
It's entirely possible that ios's shitty performance is due to me rescuing it from brick state via oven, but I don't really buy it. The phone works perfectly, except that it's very slow and experiences the occasional hang. It's. Ertainly nowhere near as smooth as my buddy's 4s. I don't expect it to be, but I do expect it to unlock without a 4 second lag.
iOS users are in the same boat, but Apple has been providing much more support to older devices.