I just ordered 8.1. I'm excited to see if there's any performance tweaks they've given it which would make Sonar run a little smoother. In the meantime, I can only say one thing. Windows 7 is rock solid, so long as you don't do anything stupid....like connect it to the Internet. But seriously, if you had a Windows 7 machine on known reliable hardware, operated by a single user, only the exact software installed that you need for Sonar, no free plug-ins from untrusted developers, and running BitDefender, I would imagine, you will be even more solid than many Mac's out there running Pro-Tools.
Most problems I've encountered were either related to possible bugs from plug-ins or Sonar. Everything else seems magically stable.