C. Ronaldo's shot is a good deal more powerful than Messi's. He's also faster, stronger, and better in the air than most strikers.
Messi's better to look at, but I think the two are pretty much at parity. Messi's also more injury prone.
And of course, if we're going through the "best player" debate again, Kaka has to get a mention alongside those two despite his recent "dip" in form.