He's undoubtedly one of the greatest forwards to have ever played the game, and as a physical specimen of an athlete he could well have been created in the laboratory as the 'perfect' striker. Convincingly one of the top 3 (attacking/goalscoring) footballers of all time, without a doubt top 5.
However, compare each of their individual footballing skills side-by-side, with one final important talent being just how much pleasure their play brings to the eye of a neutral football fan, and Messi comes out on top. Easily, I'd argue.
Ronaldo converts like a motherfucker, but rarely creates. The vast majority of his goals have been largely constructed by his brilliant teammates with him putting the big red bow on them, in comparison to Messi's conjuring of so many magical goals out of absolutely nothing. And the sheer variety of Messi's conversions, particularly in regards to set pieces, put Ronaldo to shame somewhat. He's still utterly amazing, and will deservedly win the Ballon d'Or this year, but he'd have to do more than just put goals away prolifically, whatever the size of the occasion, to truly ever eclipse a pure football sorcerer like Mess