As far as I know CDMA is better speech quality then GSM and this is due to the speech coding used by CDMA, you can check the standard of speech coding for both .
CDMA isn't a defined mobile radio standard. You should refer to a defined standard (I guess CDMA2000) for a comparison. Furthermore, the standards may provide variable rate codes (I think. at least CDMA2000 does), so the configuration and network conditions have to be considered.