The REAL comparison should be Ployserve cluster vs. a NAS cluster. At that point, it is similar. A typical NAS head is just an Intel-based Linux box that serves up files via NFS/CIFS/iSCSI.
You can do the same with a clustered file server.
The comparison comes when you get down to management. Is polyserve Windows-based? It makes it easier to deal with AD, etc. But then the cluster may be put to different uses other than a file server. You are limited to file serving on a NAS head.