I have recently leveled my 3rd 5-box team to level 70. I usually both do quests and grind.

- I do all kill x mobs quests, kill and loot head, kill x until y drops etc. Theese quests can be done on all at the same time.
- I skip allmost all loot x items type of quests. Exeption here is if its a start on a very good questchain, or if I need it for instance-quest followup
- When I have done all multibox friendly quests avalible for my level (including any prequests for instances) I go instances. Usually trying to finnish up the instance quests since theese give insane amounts of xp in comparison to normal quests. I normally grind instances until I'm 2-3 levels higher then when I started grinding. Then I find new quest-areas and repeat.

For short:
Questing is better xp than instances IF you have plenty of good quests. If you are running short on quests, or have to do quests that include a lot of running, bad droprate,loot x items etc, then you will be better of grinding instances.