Yes, I understand..

I also thought that most people will suggest the priest.

But when you really want to level fast you need a nice kill rate, too.
So you would say you think Warlock + Priest would be faster than Warlock + Mage because of..? Resurrection if I die and the healing (Warlock/Life Tap)?

For me being fast matters much more than "general suggestions". But you would still recommend a priest?

I can't decide, because it just depends on if or how often I would die - I'm sure a mage would kill faster, but the time of running to your corpse if you die also matters, and I don't think I'd die often.