Depends on what you are building? Is your build for gaming and general usage or did you build a server?
Home Server is exactly how it sounds its for a server build for server usage.
Windows 7 is one of Microsoft's best Operating systems, highly recommended upon all experts.
If you want a new OS go with Windows 8, it's faster, supports newer hardware, takes a little getting used to.
All in all its up to you. Most people on here are going to recommend Windows 7. As for me I'd go with 8.