For now my choice would be windows 10 simply because Chrome OS isn't fully matured yet. If you're the type of person who just does web browsing, email, little bit word,excel, powerpoint then Chrome OS would be sufficient for you, but if you're a gamer or who needs to use complex programs then Chrome OS won't cut it for you yet.
But I really want Chrome OS to succeed and grab a huge chunk of market share so that windows monopoly comes to an end. Chrome OS's successs would mean more affordable computers for everyone. Given Google's performance with Android in the mobile market, I have really high hopes for it.