genrally, typrobin has given a good view of KR vs CR. kalk has been known too eliminate phosphates to a certain extent to. in the end, it all boils down to what u r keeping. what is ur calcium "load" like? a KR might not be able to catch up with the calcium intake in ur tank if u hv loads of sps, clams in ur tank, unless u add calcium additives seperately. KR needs no tuning if used with a top up device like the TUBBY. but as for CR, patience is required to tune it to the optimum level u want.
juz me 2 cents.