Just my point of view...
Disc:
+ Easy to keep up dps/tank with low gear or hp.
+ Fun having utility's to boost dps(Power infusion) or to save someone (Pain suppression)
- Some mana issues in longer fights if dps is low or someone do misstakes.
- AoE healing is expensive.
Holy:
+ Ending trash pack or bosses with 80-100% mana with good grp feels satisfying.
+ Easy to keep grp topped.
- Hard to get tank/dps with low hp fast to full hp.
This is with ilvl 345.