Disc vs Holy is going to be a preferance thing. I don't have click regions - which I think is vital to making Disc work. Holy is pretty "fire-and-forget" PoM is so strong that on most content you hardly need anything other than PoM.
Renew, Holy Nova and PoH are there as gravy. On longer/tough fights I sometimes use Greater Heal.

Playing my Holy Priest Solo I've done DF groups without ever stopping as a challenge - not letting myself use anything but instant cast, and just running in a circle for the boss fights. The fact that I can do that and not even come close to wiping my group says a lot about the strengths of Holy.

Of course, the final 51 point talent really isn't worth the loss of the 21 point talent in Disc. Instant PW:S w/o CD is much better than a panic button that you'd not need if you had good planning to start with (and an Instant PW:S is more likely to save someone... and not on a stupid long CD).

For mboxing, I find the only time I'm losing health is when I forgot to cast PoM. I'm seriously going to toss it into my castsequence at some point... I just need to rethink my button layouts :P
Tier9 set bonus for PoM also really helps. Very Tasty.