Long post incoming.

Making an unbiased post based purely on actualy figures for XP gains in kills and quests, the conclusion I hope to arrive at logicaly is questing > grinding, but as I said, I'm presenting the numbers as they are for now...

Notes: "Orange" con mobs will give the Max XP per kill achievable, red will reward the same XP as a +4 orange mob
All quest XP gains listed are for same level quest, ie a level 20 quest is rated as level 20, not attainable at 20 and actualy 24+
XP for quests are averages and trying to ignore dungeon quests.
This isn't a guide, just a comparison between grinding and questing, and an outline of XP in WoW
If there is any doubt between 2 figures, or any rounding off to be done, I've done it in the favour of grinding, IE rounded UP mob XP and given lower XP numbers

Note on Questing: The XP listed per quest obviously does not take into account any of the following:
Travel/discovery XP gained
Killing mobs en route to and from the quest giver/location for completion
Grinding mobs while at said location for the actual quest (Say you need to kill 10 mobs, Sam will need 10x6)
Mobs you kill for drops, if the drop rate is high, Sam should take these as he isn't opposed to grinding a while
Waiting for a quest mob to respawn is more grinding

Reading the above, you can see questing is in fact a combination of grinding just as Sam is doing, but with added XP chunks for quest completion, the only real factor is whether or not the travel time is worth that chunk of XP at the end.

Sources:
XP formulas: Wowwiki: Mob XP Formulas
XP to level: Wowwiki: XP to lvl Formulas
Quest XP: Wowhead (filter quests by faction and level)

Lets start of with about where Sam is now, level 20, and then I'll jump to 30, 40 and 50 with the same figures.

Level 20:

A character on their own, with no outside help (or outside help of an equal level player that does not "leech" xp):

Equal level mobs: (20 x 5) + 45 = 145 XP per kill
Orange +4 mobs: [(20 x 5) + 45] x 1 + [0.05 x (Mob lvl - CL)] = 145 x 1.2 = 174 XP per kill

A group of 5 as XP varies by group size, and again no outside help:

Group of 5 - equal level mobs:
145/5 x 1.4 = 41
41 x 3 = 123 XP per mob with RAF

Group of 5 and orange +4 level mobs:
174/5 x 1.4 = 49
49 x 3 = 147 XP per mob with RAF

Average quest XP rewards vary, lets say the lowest for any normal quest is 1050 and the most is 1950 at level 20 for a yellow con quest, this is 3150 to 5850 with RAF. If you take 147 as your average XP per kill then this is 21-40 kills per quest

You will need 20800 XP to level to 21, therefore 141 mobs. As little as 4 quests, or as many as 7 (XP from "other" gains not counted, as mentioned above)

Level 30:

A character on their own, with no outside help (or outside help of an equal level player that does not "leech" xp):

Equal level mobs: (30 x 5) + 45 = 195 XP per kill
Orange +4 mobs: [(30 x 5) + 45] x 1 + [0.05 x (Mob lvl - CL)] = 195 x 1.2 = 234 XP per kill

A group of 5 as XP varies by group size, and again no outside help:

Group of 5 - equal level mobs:
195/5 x 1.4 = 55
41 x 3 = 165 XP per mob with RAF

Group of 5 and orange +4 level mobs:
234/5 x 1.4 = 66
49 x 3 = 198 XP per mob with RAF

Average quest XP rewards vary, lets say the lowest for any normal quest is 1200 and the most is 3650 at level 30 for a yellow con quest, this is 3600 to 10950 with RAF. If you take 198 as your average XP per kill then this is 18-55 kills per quest

You will need 38800 XP to level to 31, therefore 195 mobs. As little as 4 quests, or as many as 11 (XP from "other" gains not counted, as mentioned above)

Level 40:

A character on their own, with no outside help (or outside help of an equal level player that does not "leech" xp):

Equal level mobs: (40 x 5) + 45 = 245 XP per kill
Orange +4 mobs: [(40 x 5) + 45] x 1 + [0.05 x (Mob lvl - CL)] = 245 x 1.2 = 294 XP per kill

A group of 5 as XP varies by group size, and again no outside help:

Group of 5 - equal level mobs:
245/5 x 1.4 = 69
69 x 3 = 207 XP per mob with RAF

Group of 5 and orange +4 level mobs:
294/5 x 1.4 = 83
83 x 3 = 249 XP per mob with RAF

Average quest XP rewards vary, lets say the lowest for any normal quest is 3150 and the most is 5450 at level 40 for a yellow con quest, this is 9450 to 16350 with RAF. If you take 249 as your average XP per kill then this is 36-65 kills per quest

You will need 74300 XP to level to 41, therefore 298 mobs. As little as 5 quests, or as many as 8 (XP from "other" gains not counted, as mentioned above)

Level 50:

A character on their own, with no outside help (or outside help of an equal level player that does not "leech" xp):

Equal level mobs: (50 x 5) + 45 = 295 XP per kill
Orange +4 mobs: [(50 x 5) + 45] x 1 + [0.05 x (Mob lvl - CL)] = 295 x 1.2 = 354 XP per kill

A group of 5 as XP varies by group size, and again no outside help:

Group of 5 - equal level mobs:
295/5 x 1.4 = 83
83 x 3 = 249 XP per mob with RAF

Group of 5 and orange +4 level mobs:
354/5 x 1.4 = 100
100 x 3 = 300 XP per mob with RAF

Average quest XP rewards vary, lets say the lowest for any normal quest is 6800 and the most is 8500 at level 50 for a yellow con quest, this is 20400 to 25500 with RAF. If you take 300 as your average XP per kill then this is 68-85 kills per quest

You will need 120900 XP to level to 51, therefore 403 mobs. As little as 5 quests, or as many as 6 (XP from "other" gains not counted, as mentioned above)

Conclusion:

As you can see, once you take into account that alot of quests have you killing mobs, give you rewards, a massive XP bonus and that as you level the ammount of XP you need increases dramaticaly, the ammount rewarded by mobs does not: The ammount rewarded by a mob doubles more or less from 20 to 50! The ammount of XP you need is almost 6 times that needed at 20!

I can see why you'd see grinding as better at the sub-20 levels, but above 20 they increased quest XP given in a recent patch, and lowered XP needed per level.
Questing also sends you on from zone to zone with "fedex" type quests. With creative hearthstone settings and lvl 1 alts parked all over the world your travel time is cut down by alot, plus Shaman get a 15min Hearthstone spell iirc.

I did have more to add, but my lunch is ready :P

I'll add more as I think of it, please read through the above and think about it... Questing is not hard, grab Lightheaded and Cartographer adons, these will tell you where quest items/mobs are and help you navigate to them. If not use Wowhead. Grab Jamba for quest taking/finishing and other Multiboxing assistance.

Hope this helps...