Optical Toslink cable quality? Is there a difference?

None

also the key to good Toslink is the connectors

and... HDMI could possibly introduce ground issues, which an optical interface cannot

One word JITTER

If the cable works at the bandwidth required (no dumbing down or reduction of the digital data conveyed in the signal and no dropouts) then all the digital reaches the destination and digital data is perfectly preserved.

However, audio signals have timing information conveyed with them - an analog style clock signal is transmitted. It is the shape of this analog signal that can be affected by various cables and the digital data being sent.

Provided your downstream DAC device can run on its own clock asnchronously decoupled from this analog digital clock signal then you have nothing to worry about with different cables. Unfortunately many (if not most) devices run PLL or phase locked loops which are not perfectly decoupled and almost certainly may benefit from certain cables vs others or in certain lengths. The problem arises because one clock chases the other and while the secondary clock can be smoother or less jittery it can actually be worse because the jitter might even be less random.

Benchmark have completely solved this problem by doing all of the clock locking in the digital domain at 250 GHz (4 picosecond control of timing and an impossibly high speed for analog electronics to function accurately but easy enough in the digital domain). Such extremely small timing adjustments allow the rate of change in clock timing to be conducted very gently at less than 1Hz (a 1 Hz signal being inaudible and rate of change being akin to acceleration in your car. Just imagine a driver following another car and trying to maintain the same distance - a driver can choose to accelerate and decelerate rapidly in an easily perceptible jerking motion, alternatively a more skilled driver can make very gentle adjustments that passengers do not feel)

When it comes to Toslink there are many (including me) who use glass instead of plastic for a more accurate and realistic sound. I’ve used Lifatec cables for years and remain very happy with their products. Their cables are reasonably priced for the quality. You can check them out here:

http://www.lifatec.com/toslink2.html

HDMI and Toslink both the cables are used for transferring digital audio from one device to another. The main difference is HDMI cable is transferred higher-resolution audio and videos while Toslink cables are used for audio signals. HDMI replaces the Toslink. But, Toslink is also a good option because it carries 7.1 channels of very high-resolution audio.