Recently, we've added more formal uptime monitoring to our review process, and the results show that most Web hosts do an excellent job of keeping sites up and running. If they don't, they suffer for it in our scoring. Even if they get everything else right, sites with uptime problems aren't eligible for high scores. All services suffer ups and downs, sometimes for reasons beyond their control. Those sites that fail to address the problem are penalized accordingly.
The answer to this question largely depends on two factors. Are you new to server technology and not very interested? In this case, Windows is the easiest option to get where you want to go. Do you already have experience with Linux or would you like to familiarize yourself with this operating system? If so, Linux gives you more freedoms to configure your server, and you will be using an operating system that is less vulnerable to attacks from the Internet. Linux is also open source and therefore free.