I plan to upgrade to a 6 ... eventually.
I just upgraded my iPad 2 to iOS 8. It has a new feature, iCloud Drive, which is only compatible with other iOS 8 devices and any Mac running Yosemite (the upcoming computer OS). I'm sure there are other similar examples but the new version of iCloud will suffice for the point being made.
My iPhone 4 cannot run 8 and my ancient iMac topped out at Lion (10.7.x), which is three major versions below Yosemite (10.10?). In short, they can't take advantage of iCloud Drive or any other new-generation-only feature. The problem is, the 6 is mucho expensive and my 4 still works fine, thank you very much. Even my computer - slow though it is - is still limping along at a steady pace.
My goal is to get the computer done first (Mac Mini with external 3rd party screen), followed by the phone in a few months. Best Buy has some pretty good deals on fairly high quality 27" monitors, so I might go for one of them. The fact that they honour Apple's Education discounts is a bonus.
BTW, any recommendations for a monitor? I've priced out some nice IPS style units (I don't play games but do watch movies) in the $300 to $500 range. What would you buy in that range? PM me if you don't want to hijack this thread.