Is the (Shadow)Priest the better Mage in WOW?

[Skandranon]

They'll just be interrupted.  It's not the healing so much (you can use a fire protection potion and then the healing is no worse than the mage's) as the inability to use the AoE for any effective length of time.  Who cares about the potential DPS of an AoE if it can't be applied?

One of them will be interrupted. The other won't. Two warlocks is actually even more effective since the first warlock will be the only one taking damage, getting interrupted and burning mana; all the others get to AoE free.

So because the mage requires less help to deliver his AoE he is somehow less effective at it?  I'm not buying it  :unsure:

No. What I'm saying is that the max potential DPS of Warlock AoE is greater than the max potential DPS of mage AoE.

What's better though -- ability to completely and (nearly) instantly remove a target from combat, or ability to take a melee mob and possibly keep it busy?

When the mob (like all of them in Molten Core) is immune to all forms of CC, I choose the second. The best thing about offtanking is that it works on everything. You'll never run into a creature which is immune to offtanking. That makes it quite a versatile tactic.

Who'd be preferred in a party without either, a mage or a paladin?

You can't ask that question without talking about the other members of the group. If your group is warrior, priest, rogue, warlock, the obvious choice is paladin: if it's priest, paladin, druid, hunter, you might think about the mage. But in just about every case where you might want to fill the slot with a mage, a warlock fills the slot better.

Has counterspell been mentioned in this thread?

Several times. Rogues and Priests can both generate a similar effect. The Warlock's Felhunter does something which is functionally identical to Counterspell - and really, that's the biggest issue. Rogues' and Priests' versions are slightly more difficult to pull off, but the Warlock's essentially isn't.

(I'm still not buying the warlock AoE > mage AoE even though I'm not convinced by those arguing for mobility).

Well, if you won't be convinced by arguments and still won't change your mind, I don't know what else I can say. All I'll add is that it's difficult to understand the problems until you've played a mage extensively at endgame.