HDMI ARC or Optical out ?

In addition to what already has been mentioned; HDMI will transmit the greater bit rate width for some protocols, but optical is more galvanically isolated.
FYI, some of the better glass cables come from:
Lifatec. Wireworld and DH Labs. Perhaps not as critical, there are lots of HDMI cable options, though videophiles seem to like Wireworld 

I could be mistaken, but I thought there could be some compatibility concerns when using AT&T with Toslink.