I have a Pioneer DV-373 DVD player that I only use as a CD player as I use my Blu-Ray player for video. The DV-373 is connected to my Pioneer VSX-322 AVR.
I have a choice of audio connections between the two - analogue RCA or digital optical. Which is likely to produce the better sound?
It depends on how your receiver works and on the quality of the DACs (the chip that convert from digital to analog and vice versa) in both units.
If your receiver has a direct analog path for analog inputs then you should use digital optical if the receiver has the better DAC or analog if the DVD player does. This will let the better DAC handle the digital to analog conversion (but it is unlikely that the difference would be noticeable, and likely that they use similar or even identical DACs).
In the case that your receiver converts analog signals to digital for processing anyway then you should use digital optical. This is to reduce the number of digital to analog (and vice versa) conversions, since at best they are transparent, but at worst they add noise.