The stave is a lie.
To see an updated post on this topic, current as to patch 4.0.6, please read: Revisiting the Stave vs. Off-hand Problem.
WARNING: There be math ahead.
Something that drives me particularly crazy when making up my mind about a weapon upgrade is trying to figure out whether a two-handed staff is better than a one-handed weapon and an offhand. (Surely there must be an add-on out there that automatically does this for you? I’ve not found one yet, and so I always have to resort to doing napkin math on the fly.) Oddly enough, Blizzard has made this choice substantially easier for us in Cataclysm with the addition of a single enchant.
During Wrath, offhands (excluding shields) could not be enchanted. There were two separate spellpower enchants for weapons, a +63 spellpower for one-hands, and a +81 spellpower for staves. In Cataclysm, there’s no such differentiation between the weapon types, and so healers are generally enchanting their weapons with Heartsong until they have access to enough Maelstrom Crystals to switch to Power Torrent. What makes the difference, then, is Enchant Offhand: Superior Intellect.
To illustrate, take two weapons obtainable in Cataclysm heroics, as well as the Justice Point offhand. When you add together the stats on the one-handed weapon with those of the offhand, you’ll see that the stats are comparable, though slightly lower, than with a stave:
Soul Releaser (Heroic)
Now, factor in the enchant. With Superior Intellect on the Apple Bent Bough, that means you’re looking at 302 Intellect on the stave, versus 397 Intellect on the one-hand/offhand combination. There’s no comparison; the one-hand and offhand are now significantly better than the stave. Intellect is the most important stat for resto druids, as it gives us a 1-to-1 spellpower ratio (and closer to a 1:1.17 ratio, in fact, when you account for the benefits from Mark of the Wild, Heart of the Wild, and our Leather Specialization), as well as increasing our chance to critically hit and our maximum mana. If a piece of gear or a weapon has more Intellect than what we are currently wearing, it is almost always an upgrade for us, regardless of what secondary stats we may gain or lose.*
So it’s clear that on pieces of the same i-Level, the one-handed weapon will always win out. But what about when we look at an iLv 359 piece versus the combination above? Let’s take a look at Chelley’s Staff of Dark Mending, which is a random trash drop from Bastion of Twilight:
Chelly’s Staff of Dark Mending
Notice that, off the bat, this staff already has 56 less Intellect than the iLv 346 combination above. It does, however, have more spellpower. Is that enough to make up the difference? Here’s where the math comes in. Since each point of Intellect is equivalent to approximately 1.17 points of spellpower for us, that means the heroic blues would have a combined spellpower of 2194 (that’s 1729 from the weapon and 1.17 x 397 Intellect). The staff has 1955 spellpower plus 1.17 x 341 Intellect, for a total of 2354 spellpower.
So an iLv 359 stave will beat out the iLv 346 one-hand on spellpower, meaning that it will give us more throughput at the cost of about 1000-1200 mana (depending on your talents). Worth it? More than likely, yes. What happens when you combine the 346 offhand with a 359 one-handed weapon, though?
With a total of 2438 spellpower (1954 from the weapon and 1.17 x 414 Intellect), the combination is once again clearly better than the stave – thanks to the offhand enchant.
Ok, here’s the bottom line. On weapons of equal i-Level, a one-handed weapon paired with an offhand will always be better for us than a stave. If the stave is a higher i-Level than the weapon, the stave will probably be an upgrade. (This is true not only for resto druids, but also for every healer and caster in the game.)
My suggestion? Go buy your healer an Apple Bent Bough, regardless of whether you have a one-handed weapon to pair it with yet. Your Justice Point vendor sells it for only 950 JPs, so buy, enchant, and wait for a random heroic to drop what you want. I’d been running around with the Bough in my bags for about two weeks until I was finally able to equip it last night, thanks to a lucky drop from Magmaw.