I’ve got Firefly installed on my Synology CS-406 NAS box and also on my PC. The PC is a LOT faster at sending the browse data to my SoundBridge. Whether this is really noticeable depends on how big your music collection is.
Stability is another issue. I am having some problems keeping Firefly running on my NAS box. At this stage I am not sure if this is unique to my NAS or whether it would do the same on any CS-406. Lots of people are running it on many different NAS boxes with good results. The PC version seems rock steady.