Peter :I have both and they both work with a 5500 system. The 5500 is the one to get as it is in my experience quieter in operation. Both sound very good though, but I cannot tell CD players apart that well! I use the CD5500 out of preference.
When you say quieter, do you mean electrically (through the speakers) or mechanically i.e. mechanism noise?
I was lead to believe the CD5500 was the "cleaner", more "accurate" source - I knew the mechanism was more slick, quick etc but I had heard the sound was different - is this not the case?
Generally speaking, you aren't learning much if your lips are moving.