Michael Degusta at theunderstatement makes an interesting point about long term support for devices provided by Android phone manufacturers when compared to Apple's support for the iPhone.
The graph below plots the number of software revisions the phone falls behind the currently shipping OS. it's skewed slightly by the rapid rate at which Google is pushing out Android revisions but nonetheless makes a fair point.
Michael points out that Android OEMs have no real interest in pushing updates out and try to gain a sale though making you unhappy with your current phone, whereas Apple looks to gain a sale by making you delighted with your current purchase. Simplifying things somewhat if you ask me but there's definitely a germ of truth hiding in there somewhere.
The key message here is to consider the value of your phone for the life of your contract. Compare the past performance of your phone manufacturer with regards to OS updates before handing over your cash.
You can read the whole piece here: http://theunderstatement.com/post/11982112928/android-orphans-visualizing-a-sad-history-of-support