Fewer people in the zone and more mobs to grind will definitely have an effect, but the type of quests available in mulgore will have an even greater effect.
Most quests will be harder to do with tonnes of people around. Gather and kill quests will be hard, especially when the mobs are all located in a very small area.
Some quests will be easier though. Quests where you need to fight your way to a specific location will actually be FASTER with huge populations. Quests where you need to kill a single specific mob/elite will be annoying because of the population, but they will be doable from a lower level if you have the patience to wait in a 5man group to try to tag it first.
Mulgore (especially 6-12), has a huge amount of location quests which will be faster with the huge population. Rights of the Earthmother, Cleansing Wells, Venture Caravan, etc. Mulgore's "boss" quest (Fitsproket) rewards weapons, which makes it worthwhile to finish early and supercharge your leveling. Mulgore has a lot of gather quests, but the mobs are spread so evenly across the entire zone that it should not be as bad as some gather quests will be.
So yes, I think Tauren will have it easier on opening day.
I have a theory though that the fastest possible leveling will be to roll a troll/orc. After getting a few levels in durotar (specifically targeting the "location" quests (pick axe, centaur plans, mishna's skull), you swin to rachet. You get all the barrens flight paths on your way, and then take advantage of mulgore. The transit time is not wasted, because you will make that transit eventually anyway.