One of the reasons for choosing to outsource dedicated servers is the availability of high powered networks from multiple providers. As dedicated server providers utilize massive amounts of bandwidth, they are able to secure lower volume based pricing to include a multi-provider blend of bandwidth. To achieve the same type of network without a multi-provider blend of bandwidth, a large investment in core routers, long term contracts, and expensive monthly bills would need to be in place. The expenses needed to develop a network without a multi-provider blend of bandwidth does not make sense economically for hosting providers.
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.