which ever one you choose, make sure you are controlling it using the pH in the primary chamber (if you are doing a dual chamber). the reason for that is if you are controlling by anything else (pH of the tank or pH of the effluent of the second chamber) then there will be a lag time and you can drop the pH in the primary too low - causing the media to melt.

i also had better luck with a slightly oversized reactor dialed back rather than an undersized cranked all the way up.