Case x, z difference, which is preferable, why?

CASEZ :

Special version of case statement that uses a Z logic value to signify don't-care bits.

CASEX :

Special version of case statement that uses Z or X logic values to represent don't-care bits.

CASEZ  must be  used  for  case  statements  with  wildcard  don't  cares,  or else  use  of  CASE  is required; CASEX must never be used.

This is because:

Don't cares aren't allowed in the "case" statement. Hence casex or casez are required. Casex would automatically match any x or z with anything in the case statement. Casez would only match z's --x's require an absolute match.

Case

case Complete bitwise match between expression and case item expression casez enables for z values to be treated as don't cares.

casex allows for both x and z to be treated as don't cares.