This is a nice video from the guys over at Android Authority that pits the two different iterations of Android Jelly Bean against each other. At the moment it’s only Nexus devices that run 4.2, although it shouldn’t be too long before it starts to reach OEM devices. The Sony Xperia Z is pegged to get the 4.2 handset soon after the handset’s release in March, but devices from other manufacturer’s may receive updates in the meantime.
This video does a good job of demonstrating the features of each version of Jelly Bean and then comparing them to one another. I’ve been running Android 4.2 for a while now and there are a few minor differences that I prefer from the old version, such as the clock, but these are minor adjustments and don’t affect the overall user experience. One of the nicest features is multi-user on tablets, but other than that there isn’t a great deal that you’re missing out on if you’re not on 4.2 yet.