both are viable and situational imo. disc is better if your partner has a cc @70 if you are running with a dps that has little cc such as ele or boomkin I would suggest holy for chastise.
Ex. Disc/prot pally- psyfiend/dominate, scream, focus avenger, focus hoj. (Does this comp really need 3sec disorient?) probably not lol.
The bad thing about holy is going for a chastise is committing to being locked out from your instant heal for 30sec, if a team abuses that they can swap you to every time you chastise because they know you can't use serenity for 30sec.