Mitch there may or may not be a slight difference in the player's coaxial and optical output levels, and the sensitivity of the receiver's coaxial and optical inputs. If any of this resulted in a small volume difference between the types of connection, it would be made identical simply with a small change in the receiver's volume control. With the volumes being made exactly identical there would be no difference in sound quality between the two types of connection. Incidentally, the failure to balance the sound levels when judging two audio items is probably the most common reason for drawing a false conclusion that one is better although they may be in fact audibly identical at precisely the same level.
Generally the coaxial connection is a little more sturdy and might be the preferred option.
