The Drobo S is rougly 200$ more then the 2nd gen Drobo and only adds another bay (and esata and usb 3 but I doubt mac users take advantage of this)
Synology offers more for less. PC-less features saves lots of power and eliminates the need to get a mac mini to run an iTunes server.
There are major difference internally in the chipset used inside the Drobo as well as the noise. The chipset makes the raid faster, and the noise is less than the gen-2. Also, the Drobo S allows you to set dual-drive backup incase two drives fail, as well as other more minor settings such as dimming the lights, etc...it's way more than just adding one drive bay (which is also a good thing).
I have had both and can attest to these differences. The Drobo S is also built better.