there is no preference for me. they are just too different to compare.
most times, it's winter indoors and spring/summer/fall outdoors.
outdoors allows for greater range. you can hide and hope your camouflage matches your terrain. you can also stand just outside of your opponents gun range and wait for them to get up and move towards you.
indoor is all about close quarter battles. it's a much faster game. less hiding and camping out since your enemy is closer to you.