Hey John. What version of iOS is the iPod running at the moment? it can go as high as the last best version of iOS (iOS 5.1.1). If you're running iOS 4, you'll experience a big change when you upgrade to iOS 5. iOS 6 is just as good as 5, just a few minor improvements over performance (and removal of Google Maps and YouTube, that's another story).
Don't ask why iOS 6 isn't the very best version of iOS. But anyway, I see no valid reason as to why Apple dropped iOS 6 support for iPod touch 3rd gen. It definitely has the hardware to run iOS 6 (a 600 MHz processor). I guess TIME is the only reason (not the magazine, I just wrote it in allcaps to put some emphasis). It was time for Apple to support only the newer devices (as they usually do, no iOS 6 for the original iPad). However, making 3GS worthy of iOS 6 update is a good decision, but not all features of iOS 6 will be available on it. So either way, if you're on an older device and you have iOS 5, consider yourself lucky!