Basically go Holy, if you only want to heal.
And want your play to be... Target has been damaged, give them a heal to get back to full.
You get an extra AoE heal, for a slight advantage at this.

Go Disc if you want to both heal and DPS.
Your heals won't be nearly as good as Holy, but you'll prevent a lot of damage and not need to heal as much.
You've got quicker "twitch" healing too.

Both Holy and Disc are very effective healers; it's a preference call which way you go.
I suggest Disc, because you can assist with the DPS, which can be important with only two toons questing... and it won't impact your ability to heal dungeons.