I will say this, Dirk has just put up better numbers as his team's number one option, for a far longer time... for a perennial "contender," no less...
And Manu while enjoying the benefit of not having to play as the number one option for the Spurs at any point during the regular season (during his prime)... could always be counted on when it mattered the most - in the clutch... No one can argue that he wasn't the lightning plug that catapulted the Spurs to a championship in 2005. Duncan, while highly effective (as always) took a back seat during that run due the severely sprained ankle he was nursing at the time...
So one could argue that when Manu was in his prime he was an unstoppable force... and led his teams to championships... (see Argentina GOLD medal winning team in '04 and during his playoff run in '05). Manu took it hard to the rim and seemingly scored at will, or got to the line... his decision-making was impeccable. That playoff run was one for the ages as far as international players are concerned.
When Dirk was in his prime he... he accomplished... ummm... an MVP season?? A Finals embarrassment....??
You keep Dirk....
I'll take Manu.