Apart from scoring goals, most of Rooney's game enhanced the team, through balls, seeing how the games panning out, making runs to move players out of the way, if he doesn't score, he will create, or allow the creation of goals.
Apart from scoring goals, most of Lukaku's game is detrimental to the team, losing the ball from simple passes, misplacing simple passes, not releasing the ball or releasing when inappropriate.. He does score goals, but as a team we would probably score nearly as many if we had a player who rarely scored but had a better vision and fist touch up front.
That being said, I would keep Lukaku as long as poss, because despite his flaws, goals win games and he does score a lot.
Rooney every day of the week in this one though