For non multiboxers questing is always faster. For multiboxers it depends on how many annoying collecton quests are involved in each zone.

I think most people who multibox 4 or more guys do a mix. What I do is:
1 Do all of the quests that aren't Collection unless its a high drop rate
2. Do collection quests that start good chains.
3. Make up for what I didn't do in that zone in instances.

SM you can pretty much live here 30-40 except when you get bored, the xp is great.
RFD not too bad.
ZF you can do, but its not "FAST" compared to say how SM was.
Sunken Temple. Fast again
BRD/LBRS were fast on my mage team, but I havent gotten there yet on my shammies.