Disc for sure. Shadow Mend is OP. When you get kicked while casting Penance, you can still cast Shadow Mend because it's a Shadow school spell. Vice Versa. When kicked while casting Shadow Mend, you can still cast Penance and Power Word: Shield. You can move while casting Penance. Make @mouseover macros and use your favorite raid frames to become a healbot. If done correctly, you can see from up to 100k-300k total heals per BG. If you are dying, Power Word: Shield up and then hardcast Shadow Mend. Here's a 'priority' list:
Power Word Shield>Shadow Mend>Penance>Plea
Use Penance only when you need to reposition or when PW: S is down, and only use Plea to top people off, or when you need to reposition and have PW: S and Penance down.