The specialization that gives you the bonus xp perk should be the first one you go for (unless you just plain don't care about gaining xp). I'm lucky if i can play this game for an hour every other day, but i'm sittin at sr129 right now thanks to that perk and spatan ops online. ^_^
Operator is probably the most useful specialization, especially if you play BTB. Once completed, it gives you the wheelman perk. With the perk on, it will redue your vehicle's EMP stun time directly by half. So the ghost which normally gets stunned for 3 seconds will now only be stunned for 1.5 seconds, it's very very useful, and absolutely necessary to use with important vehicles like the guass hog, banshee, and scorpion. Also, it only works if you're driving the vehicle, does not work as a passenger.