we need a file server , which one should us to chose : windows server 2012 OR Windows Storage Server 2008 ?
It does depend on your requirements.We've found that Windows server is fine for up to 100 users and a single site environment (and beyond). If you then need more sophisticated management and NAS like capabilities then Storage Server offers more. Storage Server also used to support Apple (AFP) protocol. Hope this helps - maybe jot down some specific requirements.
Both Server OS are good, but I recommend Windows Storage Server 2008 R2 because it's easier for use and looks like Windows Home Server 2011.
If you don't mind the look of server 2012, go for it. If you need some legacy support (or just prefer the look/feel), I would say Server 2008 R2. It will also depend on how many users will be connected to the file server, the amount of traffic, fault tolerance (NICs, HDDs etc.), and expandability etc. Whatever you do, make sure you backup.