Provided that your HDMI cable and digital coaxial cable are both of equal decent quality, both should deliver equal quality sound signal to your AVR for what you are saying you wish to listen to.
The only difference between connecting your DVD player to your receiver with either a HDMI cable or the digital optical is that the HDMI can carry the new DD and DTS 7.1 audio formats and the digital coaxial does not have the bandwidth for that. That being said, your DVD player will not be able to output those new 7.1 formats as I think those are only native to blu ray / HD DVD players, so using a HDMI cable on a DVD player is kind of pointless...
To add to that, a bonus to using an HDMI cable is that you only have to use one cable to transfer both the sound and video signal, saving you money and wiring headaches.
Good luck.