I have a large library (17,000+ FLAC songs) that I serve to 3 soundbridges in my home using Firefly Media Server. I started out using an unslung slug, but the performance wasn’t acceptable. I should qualify that – the time it took to serve up the list of artists was unacceptably slow (20 seconds or so). Once the artist list was presented, performance was quite good (after I added an index on artists/albums onto songs.db). My second venture into NAS was on a Linkstation Pro. Hacking the Linkstation Pro and installing Firefly was quite easy (Linux noob, here) and performance was much, much better (6 seconds). I also like the fact that everything is in one easy to access box on the Linkstation.
For my particular needs, my vote goes with LinkStation Pro. It’s fast, quiet and easy to maintain.