Subs produce the sounds and pulses from moving air. So you want the air to be able to move...point them towards the back of the car, and place them against the rear seat. People have tried facing them both directions, and this way sounds better.
If you're looking for clean tight responsive bass then pount them toward the cab. If you're looking for DB, point them toward the trunk.
Con's: Learn to deal with a lot of trunk rattle!!!!!