Casex,z difference,which is preferable,why?

CASEZ :
Special version of the case statement which uses a Z logic value to represent don't-care bits. CASEX :
Special version of the case statement which uses Z or X logic values to represent don't-care bits.

CASEZ should be used for case statements with wildcard don’t cares, otherwise use of CASE is required; CASEX should never be used.
This is because:
Don’t cares are not allowed in the "case" statement. Therefore casex or casez are required. Casex will automatically match any x or z with anything in the case statement. Casez will only match z’s -- x’s require an absolute match.