HDMI ARC or Optical out ?

Optical will only give you sound, I think.  HDMI will give you sound and video.
Not really sure why you would want to do video out of a TV monitor…

From what I've read, the overwhelming opinion is that HDMI ARC sounds better than Toslink optical. 

All the best,
Nonoise

@saki70  The quality of both cable types plays a role. If you go Optical, then go glass.

I've had good results with both. There are so many variables that it will come down to your trying each one out to hear which you prefer for sound quality.

One difference that may factor in...are possible handshake issues with HDMI ARC / eARC. Sometimes (source and receiver dependant) there are issues.

All the best.

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 

 If you go Optical, then go glass.

Yes the best glass with ATT&T bnc type connections (aka Wadia and Esoteric used with their top units) with a dab of  very $$$$$  "Indexing Fluid" on each end so there's no air gap.

Cheers George

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

First , thank you to all who have responded !
I kind of thought that it would boil down to " try-and-see ."
It was worth a shot . I do have a nice DH Labs cable but not a HDMI
cable .
Marching on .

Be Safe 

@saki70   The DH Labs HDMI 2.0B Silver is a solid value and performer. 

David ;
  Thank you .

Perhaps not as critical, there are lots of HDMI cable options, though videophiles seem to like Wireworld

The thing with HDMI cables is that they are typically engineered to use the best wire for only the 4 primary data conductors (which are actually 8 wires in a balanced-data configuration).  The remaining wires for hand-shake / data / network / ARC are usually much lower quality and lower awg.  For this reason, there are many HDMI cables that perform excellent for audio/video from HDMI source, but they are not the most optimum for ARC usage (which is really a talk-back protocol).

On the current line of Wire World, the Silver Sphere looks okay because ALL wires are the same awg silver-clad OFC.  However, Wire World does not use larger awg conductors anymore.  It is likely that this Silver Sphere uses smaller 28awg or 30awg conductors.

The Nordost Hemdall 2 indicates 19 x 25awg solid-core silver plated conductors.  That's the only cable I have seen that supposedly uses high grade conductors for ALL pins of the HDMI cable.  This cable is definitely expensive ($800 retail for 2 meter), so I'm not sure if that's in your budget.

There might be others that have good ARC wire.

I have been using a Wireworld Supernova 7 glass optical with good results. It's surprisingly affordable too.

There really is not a discernable difference in the sound quality between the optical audio out or the HDMI/ARC .
 But... with the HDMI/ARC connection , I have retained the volume/mute functions on my Roku remote control . These functions were lost when using the optical out function .

Thank you to all of you for your help .
Be Safe