It's kind of a tie with me... Linux is great with security, but is rather difficult to use for someone new to the computer (due to the way it works). Any software you could ever want for Linux is open source too, so that's a plus. Windows, on the other hand, is easy to use (for someone who doesn't know what's going on or know how to maintain it anyway) and most software is made for Windows.
Linux is more powerfull in security and performance but needs knowledege of computers, linux
Windows is not as powerfull in security and performance but needs no knowledge of computers
Linux for me