I think Messi would probably be better at Madrid simply because, personally and athletically, he is a bit more flexible than Ronaldo.

Messi is an excellent midfielder and even a pivot, in addition to (obviously) being an excellent central or right wing forward. Like CR7 he is great at scoring goals and making assists, for him due to his unusually high level of accuracy from inside and outside the box. But he is also an unusually good passer and dribbler and often sets-up his own plays bringing the ball to the midfield to be distributed to the front as he runs to place himself closer to the goal. Messi gave up a lot of his goal-scoring early this season to accommodate Luis Enrique's system that needed him to play quite deep in the center, and then he gave up his central position altogether for Luis Suárez when he returned to play. All without any apparent complaint. Lastly, when it comes to individual glories, Messi seems a bit over it. But he still seemsvery driven to help his team win trophies. I think Messi would complement Ronaldo very well. Messi creates a lot of chances, some of the most in Europe, that go for nothing because he doesn't have a finisher like Ronaldo on the other end of them.

The only thing that makes Ronaldo to Barcelona more likely, to my mind, is the issue of Messi's particular loyalty and comfort at Barcelona. He has said many times that he credits the club for giving him his career by taking a chance on him when he was a tiny kid and paying for his medical expenses to be treated for his growth hormone deficiency. To my understanding, Ronaldo is a life long Real Madrid fan and that surely anchors him to the club, but he has played elsewhere.

All that said, if Messi went to Real Madrid I can't even conceive of the outrage it would foment, it would sure be violent, and not end up well for Leo.