I own the Drobo S and my popps owns the Drobo FS.
It really depends on what you'll be using it for. I chose the Drobo S since I store all my RAW photos on there and work off that drive in Lightroom. Therefore I have it going through FW800 and need it to be fast.
The network drive transfers speeds will obviously be slower; but more accessible from a variety of computers/ devices on your network.
I like the Drobo FS because I can access our media library through XBMC on my appletv without having to go through a computer as a server. XBMC can connect directly to the local IP of the Drobo FS.
However, if you have a mac mini or a box that stays on 24/7, you could hook up the Drobo S to it and just share it through OSX, which is pretty easy to setup.
Basically, if you want fast transfer speeds or only want to use it primarily from one computer, you'll get more for your money with the Drobo S. You can always add the DroboShare (
http://drobo.com/products_droboshare.html) if you change your mind.
The Drobo FS will be more convenient from the network but slower transfer speeds.