Toslink vs. Digital Coax for Long Run

Hey Squaker, I really don't think that in this situation you will notice a difference. Over 10 feet I would think that toslink with real glass would be best, but again, I don't think you will notice. In general toslink is best on long runs, especially when they exceed 20 feet.

I think a Monarchy DIP would alleviate some problems with a long cable run. Cheap enough to try, and they work in just about every application I've seen.

For short runs, I find that coax is consistently superior to Toslink. If you use a decent coax (such as RG-59/Foam or equivalent), then line losses are insignificant for digital signals.
There are two issues: (1) impedance matching, which you can verify with instruments, and (2) noise/RFI pickup.
The latter is not a major problem if you use a quality shielded cable. The same cable is "good enough" for feeding satellite baseband from the down converter to the main receiver, where signal levels are quite low...
Digital audio signals are somewhet sensitive to external noise, which translates into jitter. Some DACs are capable of re-clocking the data, making that level of jitter a non-issue, but yours probably does not. Still, if you use a quality shielded digital cable with good matching on both ends, you will retain all the information to a high degree. This is my prefered solution.