If there is a difference its probably soo slight its not even worth comparing...but, try both out if you can and decide for yorself.
That's always the best way to tell.
I use coaxile for my 5.1 surround output from my DVD player...I'm not even interested in comparing the optical output...sounds fine the way it is.
I've not been able to tell any difference from optical vs. coax for the SAME SOURCE, but I have heard differences in the same input type even from DIFFERENT TYPES of the SAME SOURCE (e.g., two different DVD players).
Try running a search on this forum, you'll probably find dozens of threads discussing this. That should give you all the info you need... and maybe more info than you wanted to know