For one thing, the CDR-220 has AUX input, and the CDR-23 does not.
CDR-23:
http://content3.eu.porsche.com/prod/911/targa.nsf/gbrenglish/comfortaudio_system
P.S. In case you have an iPod, you want the 220.
http://home.comcast.net/~kilsen/boxster_ipod.htm
The new Boxster comes with a CDR-24, and like the CDR-23, I think we're SOL for an iPod except through FM transmission or an elaborate hack. Anyone know otherwise?