IF you are on time, I recommend taking the train that passes nearer to the airside of the terminals, even if it's going in the wrong direction, because of the GREAT ramp views. The whole thing makes a loop, anyway, so you'll eventually get where you're going.
But if you're cut short on time, just take whichever train gets you there the fastest.
Supposedly, you can be from anywhere at DFW to anywhere else in about nine minutes.... if there are no lines. I always found that figure to be a bit optimistic, though.

Enjoy!