Who was the better leader?

There is no real way to qualify this and do each man justice.  They both were great leaders and utilized what they had to the most potential.  Grant had more resources at his disposal so his margin for error was greater.  Lee ultimately lost, but he new he was fighting a losing battle anyway, yet he managed to outmaneuver the Union Army for four years an accomplishment to say the least.  Now had you asked who the better man was, well that would have been easier.

What I always wondered is what would have happened had their roles been reversed? Lee commanding the Union forces and Grant the southern? Would the outcome have been the same?

Yes.