Found this thread discussing it:
http://www.serverelements.com/phpBB2/viewtopic.php?p=5602

Seems to say with a server running NasLite-2, the OS X clients saw similar peformance with SMB and AFP, and also good NFS read speed. For some reason though NFS write speed was bad. I'd say if it is an all Mac enviornment, use AFP if the server also deals with all the characters OS X allows above SMB limits. That way there won't be any file naming issues.