It's a pretty widely held view.
Letís look at this.
Left foot- Messi granted
Right foot- Ronaldo by a mile
Headers- Ronaldo by a mile
Goal scoring records- pretty much even although I think Ronaldo shades it.
Honours won- Ronaldo
Can play different positions and systems- Ronaldo
BMT- have to say Ronaldo
Taking a penalty for your life: Ronaldo
Free Kicks: Messi
Two players streets ahead of almost anyone previous to them but Ronaldo wins it. Messi is lovely to watch granted but Ronaldo is the more complete, better player.