IMHO:
There are so many potential "weak links" in a home theater system that worrying about "potential" minute variations in the digital signal between optical and coax cable from the DVD plater should probably be the LEAST of your worries!

And yes, I've tried both coax and optical and couldn't tell any difference. They both sounded equally great. Personally, I stuck with optical, just because I think it's cooler.

-Brian