Mount Hyjal has some real significance in the world and it's going to be nice to experience that. Although I imagine it will be the more popular pick and thus overcrowded.
When in doubt, assume you rolled a natural 1.
I'll be going to both zones.
I tried Vashjir first....15 minutes after I got there it absolutely exploded with people and it was taking a minute or more to loot. I decided to go do a couple runs of BRC then I went to Hyjal and by that time it had cooled down enough to where there wasn't any lag and questing could happen.