Reasons to do both starting zones before moving on:
1) They're both awesome. Considering there are only 5 new zones, why skip one?
2) It means you'll hit 85 with a lot more quests in the Twilight Highlands available than people who only did one starting zone. Continue to finish questing while earning a ton of gold!
I'm not in a hurry this time to get to max level, I plan on hitting vash first cuz it looks awesome. Once I'm done vash I will hit up hyjal next depending on how many levels I get out of vash, I mostly want to just take my time and enjoy everything on the way to 85. When I start with my alt leveling then I'll go do things I didn't on my main and rip through to hit max level.