After all, it's source code control in the end....
Clearcase has a better command line interface (and directory revisioning...and triggers..and it's easier to get code.)
PVCS has (imo) a better promotion path. No clearcase 'views' (thinking that this window his R23 code and it's actually setup to hit R22 code). A better gui (that took FOREVER to load). Did I mention PVCS is cheaper?
Of course, if I was running a company, I'd go with CVS. (But that's me)