If you are resellering webhosting to your clients then you should have both linux and windows hosting. Its true that you might be a php developer and preffer linux but windows can also support most tools that linux can support, including php, perl and other features.

Additionally some of your clients might also preffer windows rather than linux or unix. So there are two major advanges for going into windows, you broaden your service platform and you get the cool advantages of windows development.