Harder decision is Messi or Iniesta? Ronaldo doesn't enter the conversation at the moment.
How is this a hard decision, Messi SIGNIFICANTLY out assists Iniesta, Iniesta is a crap shot, the most selfish player in the team, and very over rated.
Iniesta is better than Fabregas/Xavi/Pedro going through people on the ball but he incredibly frequently keeps it and goes into a dead end looking for the shot himself rather than pass.
But for a few very flukey games his assist rate wouldn't be great this year either, this is the FIRST season he's broken 10 assists in the league, and he's never broken 10 goals in the league.
Really soak that in, this is a guy who plays the most advanced in central midfield, or as a wide striker for most of his career, and yet this is the first season he's broken 10 assists in the league... 4-8 goals and 4-8 assists a year in the league is the worst return of any half decent attacking player at Barca who plays that much...
Anyway as Bagsjoni said, the situation would be very different if Ronaldo/Messi switched clubs, not necessarily score or assist less but, Barca and Real play VERY different football, Real play direct, Barca play slow build up, this is naturally conducive to Barca passing more and Messi having more possession, higher pass completion, making more passes. Teams in La Liga(and europe) play differently against Real than against Barca, not to mention that Real excluding Ronaldo and Casillas have worse players in every other position. Using stats like pass completion and number of types of passes proves nothing except that the teams they play with play a different brand of football. Passing more isn't "better" its JUST passing more. You can tell, without a doubt that Messi at Real would have worse pass completion, would shoot more(rather than waste the pass) and would make less passes in a game.
Also anyone who says Ronaldo isn't a team player, doesn't watch Ronaldo play, ever. He constantly creates chances for his team, he's within the top 2-3 assist makers in the season basically every year, and Ozil(crap shot, slow, seemingly not that fit and unwilling to shoot much) out assists him, thats pretty natural that the top scorer isn't the top assister.
Messi would have a harder time, but not necessarily score less if he was playing with the insanely inconsistent players that are Benzema, Di Maria, Ozil, Kaka, Higuain. Fabregas/Xavi/Villa/Ibra/Eto'o are all miles ahead of those guys in terms of quality.