Messi has not performed for Argentina as good as he has performed for Barcelona TILL NOW, partly because barcelona is a much better team than Argentina (No Xavi, Iniesta)...

I think the numbers and competitions speak for themselves.
Messi has great players to help in Barcelona... whereas in Argentina he has not found the same level of support.
If Messi goes to a team where there is no support like maradona with napoli or Ronaldo with Inter then it will be harder for him. Once he does that he can be the greatest player of all time