Agree with Windows 7.

I don't think it matters which you take, with Win10 coming the end of July. That said, I would pick Win7 simply because of the interface. Win10 is bringing the Start button back, so why struggle for a month with the Metro interface of Win8, only to get our love-to-hate but used-to Windoze interface?

Whatever OS, just make its 64 bit. I have 8.1 and it works great.

The only point about Win 7 is that a lot of the touch stuff doesn't work - pinch and roll, etc.
 
But yea, I would get Win 7 and upgrade to 10.
 
@

Win 8.1 that is coming with new PC's that do not have touch screens boot to the desktop, and very close to Win 7.  I've installed several new PC's for clients in the last couple of months and they all booted to the desktop.  
 
I have Sonar Platinum on my dual boot system, Win 7/Win 8.1 and Sonar and my recording interfaces work equally well in either.  I have Win 10 Preview version installed on my backup PC and what tests I've done Sonar and my recording hardware all work OK. 
 
 

After 10 minutes with W 8.1 you forget your using it. After all I turn on my DAW and open Sonar all within 30 sec.