Well iOS in my eyes is a pretty flawless iOS in itself. The bugs and such are heavily patched up and worked on in both the stock iOS and the jailbreaking perspective, making it a top candidate for jailbreaking. But of course, you have tweaks that work only for iOS 5 or only iOS 6 and such, but there are alternatives to these tweaks in Cydia, which means compatibility isn't that big of a deal or that big of an issue.
In terms of jailbreaking availability, all iOS 5 versions are jailbreakable, just in terms of which device and iOS combination, but the biggest player for iOS 5 is iOS 5.1.1. 9 out of 10 if not 99 out of 100 iOS 5 stock/jailbroken user is on iOS 5.1.1 if you take a survey. But in terms of iOS 6, we have a bit of a limited option, considering that most devices purchased today comes with the latest iOS 6.1.3 or 6.1.4 depending on device, which is an iOS that cannot be jailbroken as of yet, and most likely won't ever be jailbroken, which is the primary reason that most users are skeptical about purchasing a brand new iOS device unless it's one of a high capacity in terms of storage. In terms of the stock iOS in itself, there's not that big of a change for iOS 6 compared to iOS 5 aside from Passbook and Social Media integration such as Facebook and Twitter directly into the iOS itself, as well as DND (Do Not Disturb) and a few other features like Siri enhancements and such.
Just remember to keep in mind that if you're planning on buying an iPhone 5, there is a chance that the device cannot be jailbroken until iOS 7 comes out, but from running iOS 7 beta 1, quite a bit of the community will opt for a stock iOS 7 considering that many of the jailbroken "essentials" are already incorporated and more will probably be incorporated as well in the upcoming beta patches possibly.