Oh, one other thing, once I tossed out iPhones, I went to Android. You certainly do not have to worry about updates rendering your phones useless in America. The carriers actively block all updates whatsoever because they refuse to update their own "control" software that they built into the original Android software that they shipped in your phone. That means unless you are running Cyanogenmod or some other custom "ROM", you will never see an update... which means that the updates do not actually slow down your phone because their is no economic incentive to do so!
That's not exactly true. Google has moved a lot of their code out of their kernel and into the Google Play Service in order to side-step the problem of carrier delays and refusals in rolling out new kernels. So if you update your copy of Google Play and related apps, you are getting a major update.
Having said all that, I did not notice any major slowdowns when upgrading my Galaxy S1 from Froyo to Gingerbread. I did notice a slowdown in bootup times when installing Cyanogenmod 7, which got progressively worse with CM9 and CM10. But CM9 and CM10 were about the same speed once loaded, and were only slightly slower than CM7. I never noticed any slowdowns with newer versions of Play Services.
So perhaps the CM team is smart enough to use per-device build options that compile code optimized for each handset as opposed to using a single kernel binary across multiple platforms.