For me it varies greatly. Sometimes questing gives better xp, sometimes it is instances that are better. I haven't done grinding yet, because I find it too boring and it's difficult to find areas with a fast enough respawn rate (at least it is for 4 characters acting as an NPC lawnmower on speed).
Instances gave me best XP when the NPCs are green, because then I can get 3 to 5 elites and area effect them to pieces(*). This gives me 1 mana break after 4 or 5 kills instead of 1 break every 2 kills if I kill them with lightning bolt only.
Quests often give me better XP, but it completely depends on the amount of running and the type of quests. I found out that the whole Eastern Kingdoms undead quests from the starting point up to Hammerfall are very good for multiboxing. Also Stranglethorn vale is very fun, because of the quests and the ganking (you basically pwn the sh*t out of the other faction as long as there are no 70 gankers). The Shimmering Flats are quite slow for levelling 4 or multiboxing characters, I skipped most of the quests there and went straight to Stranglethorn Vale.

(*) This weekend I went with an additional (druid) to Scarlet Library and we could just pull all elites in those chained rooms towards the boss (6-8 elites at once). We were about level 35 then. We were quite amazed that we survived, didn't expect that (because all these elites hitting on leather is ... heavy).