A 2-class group: who would you pick?

[Concillian]

Personally I would choose warrior / shaman

1) If you don't want to be frustrated in pickup groups in instances 1 HAS to be a warrior, provided you understand how a warrior functions in terms of holding aggro of multiples.
2) a shaman can heal well enough to be primary healer if they are properly equipped and spec'ed. I've seen shamans primary heal through 40+ instances, though I know they won't reliably get you through 55+ instances.
3) If you are lucky enough to get another healer in an instance groups shamans make damn good off-tanks and you will have NO aggro problems whatsoever. EVER. Which means even if the healer you pick up is spaz-tastic, they will have no problems keeping your group alive because either the warrior or the shaman will have aggro.

My experience in playing a warrior through 40 and playing in lurker groups with my priest is that if there is a good lock on aggro, only mediocre healing is necessary. Healing becomes an easy job and a pickup healer will work just fine.

Any duo will rip through 'solo' content pretty quickly, but IMO the warrior/shaman is best equipped for instances. Warrior/priest is good too, but IMO not quite as versatile. A warrior/priest has no defense vs. an overzealous rogue or mage destroying the group, but a shaman can pretty much out-aggro anyone on single targets, so the warrior/shaman give you total aggro control in terms of a warrior (multiple elites at once) and shaman (best single target aggro magnet). Plus you have the versatility of a healer in the case you can't find one to go with you.

Priest/mage is without a doubt best equipped for solo content with the AoE ability + healing + plenty of drinks, but you will have a lot of problems in instance groups, because neither of you will be able to tank, you will be at the mercy of a pickup player for aggro control, which will often be more miss than hit