As with a Bear/Cat dual spec, these two dovetail into each other a bit more naturally than other options (though not, for obvious reasons, to the same degree that feral always has). You can often recycle weapons and non-set gear between them, and either way, you should be used to occupying a ranged position and familiar with positional requirements.
Pros: Possibly the best overall choice for a player who prefers to stay ranged and, as stated, a lot of gear is easily swappable. If you usually heal in raids and want a quick swap to a much higher-damage build with minimal gear changes, this is it. If you usually DPS in raids and want a quick swap to tree to get that heroic off the ground with minimal gear changes, this is it.
Cons: Not many, although you'll (again) want a different gear set for each role. +Hit is useless for Restoration. +Haste is less so, but still not amazing compared to raw +spellpower. +Spirit, while not useless to Moonkin, is generally less useful than more offensively-oriented stats, but casters are also going to be affected by upcoming changes to mana regeneration.
Build both sets now; you may find yourself swapping in a few Resto-oriented pieces if mp5 takes a nosedive in 3.1 (although I doubt it's going to be that bad).
RESTORATION (PvE) and FERAL (BEAR): WHO IS YOUR GOD NOW?
Otherwise known as the "One-Stop Shop for Complete and Total Ownage of the PvE Game."
This is the option I find most tempting, if only because I spent a mind-boggling amount of gold on respecs between feral and resto in BC. Additionally, the majority of bear builds allow for decent Cat DPS; this may be the only dual-spec in the game where you could conceivably tank, DPS, or heal within a few clicks. If your guild's going for an Immortal or Undying run, or if you're on progression content without the need for an additional tank, you'll have your choice of whether to help out as an extra healer or extra DPS. 5-man runs would also become less of a nuisance. Murphy's Law dictates that the moment you respec into Resto is the moment that all of your server's tanks suddenly vanish.
Pros: Able to cover the two most in-demand roles for PvE content while still putting out acceptable Cat DPS. Need a tank? Sure. Need a healer? Sure. Need DPS? Well, you'd be better off taking someone with a balance or pure cat spec, but you should still pump out a creditable level of damage.
Cons: Once again, you're tasked with building an entirely different gear set for each spec, and the bear/Resto build bears the distinction of being possibly the only dual-spec in the game that should realistically expect to make use of three gear sets.
RESTORATION (PvE) and FERAL (CAT PvE): WHEN TREES ATTACK
This is a version of the aforementioned Feral/Restoration dual-spec, but with more emphasis being paid on on the melee DPS side of the Feral tree. If you don't like tanking and your raid can comfortably accommodate a Cat on fights where you really don't need to heal, this is probably the way to go.
Pros: Pretty much the same as bear/Resto with the flip side being that your DPS is better. If you use Feral as a means of grinding and getting dailies done and are too in-demand as a healer to realistically expect to tank much, this is probably the way to go. Why sacrifice damage to a build you're never going to use?
Cons: The whole "I'm never going to use that spec" doesn't always last long, and weaker tanking capabilities may eventually be a nuisance or a hindrance to getting groups that already have a healer while you're feral.
RESTORATION (PvE) and RESTORATION (DREAMSTATE): WHEN I WAS A SAPLING, WE DIDN'T NEED WILD GROWTH, JOHN BOY
I'm going to make an exception here to my earlier promise that this would mostly stay a discussion of swapping between different roles, because: a). Resto is really the only spec that can use an almost entirely different build to do the same job, and: b). With the upcoming changes to the 5-Second Rule and mana regeneration in combat, a Dreamstate spec may very well become more attractive.
Pros: The ability to swap between two different Restoration builds depending on the fight could very well be invaluable on upcoming content. Make no mistake -- Druids are still about their HoT's, and Dreamstate is a spec that tends to become progressively more unattractive as your gear gets better. But HoT healing is oriented toward use of the 5-Second Rule, and that's going to take a hit. How big a hit remains to be seen, but the additional mana regen you get from Dreamstate is an attractive proposition, especially on lengthy fights.
Cons: Any way you slice it, you're still going to have to respec unless you enjoy grinding as a healing-oriented toon. Dreamstate has some limited offensive capability out of the Balance tree, but not really enough to make a huge difference while you're doing dailies.
Patch 3.1 brings us Ulduar, dual specs, significant changes to all the classes, and more! We've got you covered from top to bottom with our Guide to Patch 3.1.