If there is one thing to hold against Messi, it is that Ronaldo has a more extensive career in that he has been the PL's and the Ballon D'or player of the year whilst playing for United, and he's done the same at Madrid. In addition, his CL stats are off the charts. Messi has been good in the CL, but not Ronaldo good, which one may argue further proves that Messi is best suited to the Spanish league and that Barca team. He isn't carrying this Barcelona team to the CL final, and he isn't carrying Argentina.
Ronaldo, one may argue, carried RM to CL victory last season, and is on track to doing the same this season. In addition, he has a slightly better record for his national team, although I would be hard pressed to argue he dragged Portugal to victory.
Messi is the better player in Spain, but his he a better player everywhere else? There seems to be evidence to indicate that is not the case.