Each ferry company releases ferry schedules at different times. Some are known now, but some maybe about a month before or even less. So to get the full picture means waiting until late March/early April. It's near Greek Easter weekend which will mean alterations just before and after.
Symi is the most touristy with the most ferries. Kos would have roughly the same number of daily ferries.
Though you say you have restricted time you don't necessarily have to return to Rhodes. For example Kos has just as many connections ferry/planes back to Piraeus/Athens.
Symi itself 2 connections weekly with Piraeus, I suspect you could one of those services.
But not knowing you overall plan and what has been fixed, I may well be talking rubbish.
My recommendations involve more time, which would be Tilos or Nisyros. Realistically it would be a choice of Symi or Kos. They are all good choices but it's down to your time factor.
Edited: 12:41 am, February 19, 2013