I would divide into 3 main criteria to decide who is better:
1. End products (direct impact)
- no. of goals and assists
- importance of their goals and assists
2. Performances (indirect impact)
- how well do they perform on the pitch as related to their given role (strikers, wingers, playmakers, midfielders, defenders should all be judged differently)
- quality of their performance
- influence/impact on pitch
3. Achievements (what it matters)
- how successful are they in terms of winning trophies for their team (WC, CL, league titles all counts)
- whether they overachieve or underachieve in respect to the actual level of their team
For 1 alone there's no question both are worthy to be claimed as best player, but since both are equal we need look further to 2 and 3.
In terms of 2, Messi is simply better but Ronaldo isn't as bad as many here think he is, as he is only a striker (or wing forward) and you can't expect striker to go playmaking by himself. He already does his striker role the best (keep running into good position, shown aggressiveness and determination to always find ways to shoot/score, never give up, has been his team driving force up front, deadly in air and inside the box, unplayable in counter attack situation etc). But then Messi is simply brilliant in both goalscoring and playmaking role so there's no contest here for me.
Now with Messi better in 2 and equal in 1, the only chance Ronaldo may turn it around is down to 3. Ultimate purpose in football is always about winning trophies, there's no use if you keep outperforming everyone else but you keep failing to win any trophies (except your teammates are really shite). Now if you could lead a poorer team to great success (aka Maradonna) it will be even better. Pele won't be the same should he failed to win any WC. Di Stefano won't be the same either had he not won any of his 5 European Cups with Real Madrid. If Messi lead Barca to double this season (which is likely) there will be no debate anymore regarding who is the best (this year and over their career so far)