I recently picked up a Sony CDPCX455 400 Disc CD Changer that unfortunately won't play MP3 files through the optical connection. Since I found this out before I opened the box I still have a chance to bring it back.

Is there really a noticeable difference between using the optical over the L/R RCA connections in this case? I will primarily be using this player to primarily play back standard audio CD's and occasionally ones with MP3's.

I think MP3 will be the limiting factor in the sound quality, not the D/A and audio circuitry on the changer. I can't think of a time I've ever run into someone commenting on the quality of their player not being adequate for MP3.