If you are into pvp, realm can be a consideration too.
The side with more players, generally wins most of the Wintergrasp battles.

If you want to world pvp, you'd want to be on the side with fewer toons.
But not to the extent that you have no chance.
Something like 45-50% vs 50-55% might work.

If you do a lot of battlegrounds, you'd again want to be on the side with fewer toons.
A queue ends, and the BG begins, when both sides have the maximum number of players in the queue for that BG.
That's not to say all of them will click to enter, but if their side has 200 people queuing for a 40-man, and the battle begins as soon as your side has 40 people queuing, it will be much shorter waits.



PvE considerations are more along the lines of...

Are you into raiding?
How many progression guilds exist on the server and on your faction?

How mature is the auction house?
You want a balance here, between things are available but rare items still sell well.



Do the stress test on the realm.
Try your team in Dalaran if you can get a port.
At prime time, and see how it runs.
Anywhere else will be smoother.



Check into the aspects which are most important.