I would go with what fits the easiest for you. You see 'em facing out alot. Shows off the bling audio brand names on the voice coil dust covers etc. If they face in you could actually use the trunk and not worry about tearing the cones !
You are just moving air. The longer the voice coil excursion specs of the speaker the more you will feel it. If the enclosers are not ported facing back may make for more boom.