I have both. I have tanked and healed for both.
With the new changes, they both bring utility and CC.
Depending on spec, there's some serious DPS flying around for both.
If you are starting from scratch, and are planning to solo, I'd suggest warlock, just because there's less downtime.
I know everybody is pointing at portals. Seriously, how important are those? With a group's ability to use the DF to get someplace (and back again), a warlock's ability to summon, a meeting stone, flight, 30 min hearthstones, it's a nice to have. I don't know that I'd call it a make-it-or-break-it decision on group comp.
Which one strikes your fancy? Which one feels better (fits you best) and does any of the new race/class combo cause you to lean one way or the other?
Would love to have a magic 8-ball that lets our guild know which class/group combo is gonna rock in the different instances, with the new CC, and which one will be generically "most wanted". It's just too soon to tell, and I suspect there will continue to be adjustments to both classes and instances after Cata comes out. We have 3 warlock players in our guild, who LOVE the class, love the playstyle, and that is what they are sticking with.
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There are those who play tank classes, and those that tank.
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