I don't have a CA reactor, but the common concerns include.
Purity and at what PH level does the material start to be effective. A high quality Media will have few impurities and not require as low of a ph as others.
I just finished researching some infornation on the subject and have decided to use GEN-X CARM. It is a german made media, does not pack tight and dissolves very good. my 2 cents worth. I have tried others with a tighter pack and did not like the results.