Keano66
I use Win 8.1. In fact, I have just done my annual OS reformat with a new 8.1 system. I did try Win 10. It's ok. But the downside was the update process. Windows 10 takes over your computer with it's updating process whereas 8.1 means you can update when you want (or not at all!). I have never had any trouble with Win 8.1. I will never go back to Windows 10 - I view it as a massive spying OS.

My thoughts exactly! Windows 8.1 is very fast, smooth and trouble free.

Cactus Music
You can't even buy Windows 8.1. .

Of course you can.  Plenty of legit, sealed old-stock packages on Ebay.  Just make sure the seller has good feedback.
 
That said, and even though I'm a happy Win 8.1 user (Win 10 too intrusive/quirky for my tastes), OP needs to do his own research and figure out what's best for his situation. Audio interface drivers, and the fact he's stopped at Sonar X3 being factors at the top of the list.

#1 priority is making sure ALL of your hardware has Windows 10 drivers.  This is your computer, audio interface and any other peripherals. If they don't, you could develop a lag because Windows 10 put the best driver IT thought was best. I've seen it happen.
 
If all of your stuff has Windows 10 drivers, it is a much better OS IMO.
 
On my studio computer, I am going to wait until my audio interface putzes out before I upgrade.

Thank you all for your input.
 
Happy and Healthy New Year.
 
Gerry