Asif and Aamer are really World class but here Salman Butt is not in a mood to compare them with any other World class bowlers, he said this only so that A&A can get more confidence from this.
About on going discussion I will rate Aamer and Asif higher then all other nations opening bowling combination and their are quite lots of reason for that.
Aamer is much more young then any other international bowler but he is far more intelligent with his line and length, pace variation, swing variation etc etc.
Aamer is left hander so combine him with Asif is far more penetrating the Morkel and Steyn.
Asif on the other hand is more sort of irritating bowler, he will bowl medium pace but in one line which obviously irritate you as a batsman and eventually you loose your wicket.