Ronaldo is better in my opinion because he is more complete. Ronaldo has carried every team he's played for and scores 25+ goals a season no matter what.
I don't think the same can be said for Messi. You could put Ronaldo in any team and he'd score 20 goals a season by himself. He creates his own chances as well as other people's chances.
Messi is a team player, but if you put him at a lesser team he wouldn't be as effective as the players around him wouldn't be as good. He needs the players around him to do what they do for him to be effective. In my opinion he wouldn't score 25+ goals a season without the players around him doing what they do and being the best in the world at what they do.
Until Lionel Messi comes to the Premier League and carries a team like Ronaldo did, I will always say that Ronaldo is better than Messi.
Both are out of this world, but Ronaldo a bit more so imo.