Outland has a fairly linear zone progression. From Hellfire Peninsula, you move on to Zangarmarsh, then Terokkar, Nagrand, and Blade's Edge. After that, though, you have a choice of two zones: Netherstorm or Shadowmoon Valley. They're both of approximately equal level, and on both the characters that I've taken through Outland, I've only needed one of the two zones to get to level 70. I've picked Netherstorm both times, and was 70 before I was done with it. That leaves the Shadowmoon quests available for lovely gold rewards.
Having quested around in both, I have to say I vastly prefer Netherstorm. Shadowmoon Valley just feels icky to me. The mobs are constantly killing me, and the zone just doesn't seem to "flow" like it should. Also, I love the way ethereals look, and Netherstorm is full of them. However, I know people who feel precisely the opposite way, generally preferring Shadowmoon and avoiding Netherstorm. What do you think? Do you have a preference between the two zones, or do you like them equally?