If you are only running a single MAC, then I’d agree that you want the S. The NAS is much better in an environment where you have several pcs or don’t want any dedicated pc to host your drobo. In my case, I wanted the flexibility and features of the FS and was willing to sacrifice any potential performance differences.
My FS is on a gigabit network and I did some rough estimates a few days ago from my Windows 7 Pro laptop. I was getting reads up to 55 MB/second and average writes around 26 (peak 30) MB/second which is not too bad.
I am using samba on windows, but for anyone looking at the FS it also appears to include netatalk for Mac users so you can connect as an AFP fileserver which may have some better and/or different results.