It's been my experience that SME as a *file* server is as fast or faster on equivalent hardware (mostly faster) than Windows 2003 Server. However, Shadow Copy ("Previous Versions") is so compelling for end users and for me as an administrator (Users can recover their own files, overwritten or deleted, without involving me!) that I always recommend Windows Server 2003 for machines that will be only file servers. (And I still don't expose Windows machines directly to the big bad Internet.)
Just my two cents' worth.