Blizzard designed two starter zones for Northrend, unlike the crowded starting area of the Outlands. They are essentially the same in terms of number of quests and mobs, so generally the experience gain rate would be the same. It boils down to a choice of doing much of your adventuring in a snowy, dreary, uh, tundra or a lush and verdant, um, fjord (never mind what Wikipedia says, the staff of WoW.com are mandated to pronounce it as fah-geord). Personally, I went for the fjord to buy WoW Gold.

Note that skipping the Borean Tundra means skipping the D.E.H.T.A. quest line entirely and missing out on some Achievements. But that is alright, those guys are self-righteous, anyway, and a true servant of the Light can not be distracted by some silly animal rights crusade! Of course, the reality is that the Borean Tundra also gives prettier shinies, such as the Axe of Frozen Death for leveling Retribution Paladins. There are also a variety of rings and necklaces, all awarded by quests in the area about buy WoW Gold. The Howling Fjord has less cool stuff to buy WoW Gold.

It used to be that zones were attractive because of the number of Undead and Demon mobs, but since Exorcism is now an equal opportunity whacking spell, it does not matter so much. The only bonus is the chance to use Holy Wrath more often, and in this case, those opportunities are about the same about buy WoW Gold. Finishing all the quests in a zone will grant about a level and some change. Do both zones to squeeze the most bangs for your leveling buck. And holy crap, guys, I am not even taking into account Glyph of Arcane Blast here. 18% damage increase per stack? Multiplied by the power of awesome? That equals yay. Yes, indeed.