Much appreciated advice thanks!Monk is pretty stupid OP and it's a shock they've mostly avoided the nerf bat so far. Its only real weakness is its mana consumption, which very rarely matters in M+ since you can just drink between pulls.
As far as holy/disc in M+, it basically comes down to this: Holy is a very easy healer but does poor DPS, Disc is a strong DPS (at least for a healer) but harder to heal with. (And yes, you'll be spamming a lot of shadow mend.) If you're doing keys sufficiently high enough* that your damage matters, roll a disc. If not, roll whichever you prefer. They're both perfectly viable as healers in keys, even if Disc is a bit harder.
* For your group. If you're sufficiently low enough or your group just isn't good, any key level could benefit from your DPS.