You are really setting your side up to fail, lol! I would absolutely love to downgrade to iOS 4 on both devices, but as you should know, can't. Glorious Apple has decided that nobody, including the Apple "Genius'", can downgrade their own devices firmware. Yet another reason I left Apple for good. As said before, I spent $500 for 1.5 years good use, which is totally unacceptable to me...
Well, I tend to research what I'm getting myself into and have saved my SHSH Blobs should I need to downgrade. It's not impossible, it just requires some planning.
I agree with you though that Apple should have never made that call regarding downgrading. Though, even if you aren't going to go through the trouble of saving SHSH blobs, it's still important to stay away from adopting a version of iOS until others have reported back on it with your device. This saved me a lot of headache when iOS 4 was released for the iPhone 3G. Apple hasn't been too good about this either. There's really no reason versions of iOS are even released for devices that simply can't handle them.
I'm still not stepping back off of my statement though that Apple products pay for themselves. With the proper planning and some research just about any iOS device should last a long time.