Digital rca cable will always beat out optical soundwise but optical precludes any ground loop issues common from cable boxes and such.
When you connect your CD player via digital out you essentially by-pass its DACs and use the DAC in your pre/pro. If you use the analog rca, you will be using the DACs in your CD player. So it becomes a matter of which componet has the better DAC and which sound you prefer.
On Blu-Ray DVD, you must use the HDMI to get the new HD audio codecs inhernent in these recordings. If regular DVD, use can use the digital audio out to your processor. I run the component straight to the TV as I don't have HD/Bluray DVD and the digitial audio out to the processor. Same with the TV, I run an digital from the cable box audio out into the digital audio in on the processor and high end component cables from the cable box to the TV.
Most all newer LCD/Plasma have an digital out but it is optical and the HDMI output for the Bluray audio codecs.