It is very difficult to compare, as then and now are very different. You watch the 86 'hand of cocaine' game, and an England defender cuts him in half, what would and should be a red card today. Diego 'crack pipe' Maradona grew up playing in a more physical league where "revenge was got in first" and that carried on in Spain and Italy. Messi is protected now, the little drug mule must have had knees of steel and calfs of tungsten to survive, not seen Messi dropped once properly. Protected (Like we saw every week with C.Ronaldo in the PL) in Spain, which is a bit like Scotland in that it is a two horse league and the difference in quality from top through the middle to the bottom is so vast I suspect the Barca/Real B-sides could do the business versus 50% of the league.
Lets say Messi inspires Argentina to the World Cup this summer, admittedly it will be some feat with that dope fiend hep C'd to the eye balls maniac in charge, but can it compare to winning it almost single handed with the physical rigours and refereeing disasters of the 80's for instance.
End of the day, so long as he keeps producing, in years to come and the decades thereafter Messi will be revered and saluted along with the best there have been and pass into folklore. Shearer is a modern great, Dalglish a wonder of the 70's, our (and the) Holy Trinity of the 60's. Every generation has its icons, Messi looks to be going on to cut his path in the history books, shame it is for such a team of cheating, diving, handball (Henry!), fake card waving snides to ever proclaim themselves a cut above.