Here are a few ways to consider it:

If you love upgrades on a continuous basis, keep doing Outlands quests. You'll see upgrades, and get xp.
If you love doing quests just for storyline, do Outlands quests.
If you want achievements, keep doing Outlands quests.

If you want fast xp, go to Northrend.
If you want big gear upgrades, go to Northrend.

If you like quest storylines, and gear, but not so worried about xp - go to Northrend, do some quests for good gear upgrades, then head back to Outlands to continue questing. But if you do this, I'd bet that most people will just stay in Northrend, because the gear, gold and xp are so much better...