Masshole has some good points. Statistically Messi doesn't does as well for Argentina as he does Barca, but there's not been much difference in the level of his performances, just the stats (from the games i've seen). In order to be classed as doing something on an international stage you really have to win the world cup, which both Di Stafano and Puskas didn't do. I know puskas record is insane in the 50's and 60's, but i just can't see him being as effective as he was then, if he played just now.
Also, as you said he does have two of the best ever midfielders helping him out, but he has scored 50% of Barcas goals in 2012. He's also set up a lot of his goals and others by picking the ball from just over the half way line. Technically Zidane is arguably better than him and played in a completely different position. Honestly, i couldn't argue as he was one of my favourite players to watch.
Anyway, it's pretty much impossible to compare players from different positions that played in different eras. Thanks for the input, haven't heard the name Puskas or di stefano mentioned in a while, clearly know your football.