They are both beneficial for chest, triceps, and shoulder development. The flat bench tends to recruit less shoulder, while utilizing the pecs and triceps as movers. The incline, in addition to pecs and triceps, will also recruit the front deltoid head.
Some will tell you that incline hits the "upper part" of the pec. This is erroneous. The chest fires as one unit whether you push horizontal or push incline. The difference comes from the degree to which other muscle groups are utilized to complete the movement.
