Def. questing but Grind on your way to the quests. that way you knock out both "questing" AND "grinding"...makes sence right.

you are grinding, but in the same time, you are picking up items that you need for quessts...

i found this to be most effective for lvling. i have only had my warlock for about 2 1/2 weeks and I am a 37. ""(just shy of 38)