In my opinion:
1. Windows 7 is a better OS than XP, and for me to say that is a lot, because I was a DIE-hard XP Pro fan. Windows 7 is very stable, and more drivers seem to be available for Windows 7 than for Windows 8, although that will change with time.
2. Windows 8 has some internal changes that some folks have lost their minds over, and some folks have gone to great lengths to install third-party software to recapture familiar methods of doing things, like keeping the old Start Menu, rather than the new user interface that comes with Windows 8. That being said, as one person above posted (and this is my approach too), I have decided to go 'whole hog' with Windows 8, and have learned how to work with the changes in how things are done. I have found that in Windows 8, with things set up the way I like, that I can fly through things now with greater speed than I could in Windows 7. Those issues are largely a matter of personal preference. The REAL thing to look for with Windows 8 is to make SURE that any software drivers that you need for any external devices EXIST and are STABLE (midi controllers and synths, as well as audio interfaces). Do NOT assume that things will simply plug in and work.
So, do your homework on your gear that will be connecting to your computer for EITHER Windows 7 or Windows 8, and other than that, choose the one that you like the feel of.
A great many folks here in the forum are using Sonar on Windows 7, and a great many are also on Windows 8. There are, however, a small number of folks who have upgraded to usually Windows 8, only THEN to find out that there are no drivers for their external gear. (Those folks are weeping and gnashing their teeth and pretty much not happy campers).
One additional thing to tuck away in your brain somewhere is that a piece of freeware called DPC Latency Checker, which does a nice job of presenting a scrolling graph of detected system latency, does not report the proper times in Windows 8. It works fine in Windows 7, but the kernal timing information has changed internally, and to the best of my knowledge DPC Latency Checker still does not post proper latency values. There is, however, an alternative piece of freeware, called LatencyMon, that works fine in both Windows 7 and Windows 8, and it does a mighty fine job of posting similar (even more detailed) information on your system's performance and latency and such. So, just remember this, and whenever you get a system with either OS on it, DO download and run one of these tools to give you an idea of whether or not your system in its state at the time of running the tool - can adequately handle audio streaming as done with programs such as Sonar.
The BEST of it all in your new adventures with operating systems and all that goes with it all.
Bob Bone