I considered WHS, it's gotten so much praise! Instead I'm actually running XP Pro on a spare laptop as my 24/7 server machine for my home wifi and Ethernet network. I set up sharing the USB external HD and mostly it's working out. I have an app that doesn't see the data sometimes and I have to kill it, reestablish the connection from client to host machine via poking around in Explorer, then restart the app. I suppose another OS might fix that, but I can live with the occasional inconvenience because I know how to fix it. Not sure what OS would alleviate that problem...Windows Home Server is a really neat OS.... As long as you do things the way it wants you to do them.
If you don't really need server-specific needs (eg running a domain, acting as an email server, easy remote access outside the network) Windows 7 would probably do what you need.
For me, the main advantage of WHS 2011 is that it will automatically backup my other Windows based systems including laptops wirelessly as long as I sleep or hibernate them. It does this by default between midnight and 6am I believe. I'm also running Stablebit drive extender as it makes adding capacity very convenient.