You can comfortably pull multiple mobs and AOE them down, versus feral & balance which will not be able to handle as many 86+ mobs at once and both DPS specs are built for ramp up damage, which typically is slower for quest mob grinding.
Plus you can more easily queue for dungeon tanking in the same spec & gear.
If not, Balance 100%. I can kite for days, whether it's 1 mob or 20. I know Balance has some ramp up, but dotting and shrooming and using every Shooting Stars proc seems like a great way to handle as many mobs as you can keep in the mushroom patch.
From what I've seen past 2 days Guardians are doing kitty level DPS with 10x the survivability.
In later zones, this is definitely true, especially in mob-congested areas. Been trying to finalize some things on the beta before MoP goes live, and I've been helping some guildies optimize their questing pathing... and redoing some of the zones reiterates one thing: it's not uncommon for the mobs in quests to be a higher level than you.
Starting in the Jade Forest, you get a false sense of security since the mobs are lower level than you (and you'll likely outgear the place). While it does happen to some degree in Kun Lai, when you hit 88 you're going to notice most of the mobs you encounter are a higher level than you. Actually, in some of the rougher areas, not only are all the mobs lvl 90 with ~400k HP (while you're still 88), but it's nearly impossible to not pull a group. Also, some mobs can be deceptive to the point you think you can just round them up and AoE... until you realize you got a stacking nasty DoT that would even kill a tank or they possess a mechanic that makes your life miserable. Even as a tank, I found it rather easy to die if I wasn't careful... in general, I tend to limit the AoE pulls to 5 mobs in later areas, because with NS + HT and Typhoon and Ursol's Vortex the situations can still get rather dicey. Part of the problem is that some congested areas limit how much you can actually kite.
Yes, I've done some of the questing as Feral as well, and as long as you stick to single mobs you're fairly safe. In early zones, AoEing and surviving is a piece of cake. Later zones... well, even cleave situations can be rough. If all the mobs did just melee, things would be easy, but a fair amount of mobs cast some pretty nasty spells that will make quick work of a Guardian if not dealt with properly. I suppose the simple version is that our avoidance is so low as Feral (and even passively as Guardian) that all those little melee hits add up, and all the special abilities and spells just amplify the problem while level.
In all honesty, I hope when MoP goes live that the difficulty of the later questing areas does not decrease, because I think it really promotes people learning to play their classes and potentially grouping up (and using those talents that are CC/utility-based). I think if people hit 88 and leave Kun Lai, they may be in for a rude awakening with the first quest hub if they treat it like the previous zone... it's always fun watching opposing faction tanks die while trying to AoE down a group.
*edit* - Perhaps I should give you what I find viable... DPS spec from 85 through 87 clearly wins, 88 and up is a toss-up between DPS and Tanking. Balance may have it slightly better than Feral in later levels, just for the sheer fact that stats scaling hurts for energy/combo point generation. Honestly haven't leveled as Balance, though. At level 88 and higher, some quests are faster as a DPS, and some are faster as a tank.