Messi's a far better technical footballer than Ronaldo, but he lacks the drive Ronaldo has.
How many times have you seen him get the ball in a good goal scoring area, but rather than shoot decides to pass it off to a team mate to either let them score, cross it, or initiate a one-two. Ronaldo's far more direct in that sense, if there's a chance he can shoot, he will. The team know he's the hitman, and they play to his strengths, somewhere along the lines Messi or the Club have decided Messi needs to get the best out of everyone else rather than the other way around.
His off ball movement is also piss poor at times too. Ronaldo is always on the move, even if he's not as quick as he used to be. He'll try and get behind the defence or make the movement to make himself available. Messi can often be guilty of walking around the pitch without a care in the world, or just wondering behind a cluster of the opposition players to prevent getting the ball, in the end he drops deeper to pick it up off the defence.
In my lifetime Messi is by far the best player i've seen, but sometimes his laziness/lack of will to run infuriates me. So good yet almost seems like he's not trying half the time.