Compuserve classic is the original compuserve which has been knocking around for ages and compuserve 2000 is a new version with different access numbers.
If your choosing between the 2 then go for CS2000 as they are phasing out CS Classic and slowly getting rid of the numbers.
According to Internet Magazine (May 2002) AOL are axing Compuserve 2000 in June. The classic service will contiune. "But as the classic service has a tiny membership, the reality is the move will mean the end of the CompuServe brand in the UK"