Holy Weapon ChoiceFollow

Whichever gives you the most stats. Generally staves are easier to find because a staff is just 1 item with a statboost, whereas if you'd find a nice dagger you'd still be stuck trying to find an off-hand item to go with it to make the total statboost better than what your staff gives.

I think Blizz wants us to focus on 1-handers + offhand items though, since if I recall correctly there isn't an arena healing staff; just a mace and offhand.

At level 70 there are no healing daggers, the choice is between a staff or mace + offhand. All healing classes can use maces, generally the mace + offhand will give more +heal than a staff of the same item level. Staves can't be used by paladins, and generally give more mana regen than a mace + offhand of the same item level, there are a few staves out there with huge amounts of spirit that only priests and druids benefit from. For PVE ideally you'd have both, and switch weapons depending on how important mana regen is in a given fight. Mace + offhand combo is the general choice for PVP, they can be used by all classes, mana regen isn't as important in PVP and that's all you can buy from PVP vendors.