Kollywood says Illayaraja is a born composer, and rahman is not that good a composer, certainly not by his natural gift, but he is a very good arranger and an excellent engineer. Meanwhile Illayaraja has is an immense gift, and it is as rare as finding a polished diamond as soon as it is mined. Furthermore there is not much in Raja's music that sounds aged, because of that one component called 'melody', that is very much a composer's domain.
Illayaraja displays both these qualities of extreme softness and ability to absorb. Meanwhile rahman is humble, but that is probably because he knows what he is not. Moreover the greatness of someone's music objectively by how it surpasses generational boundaries and Illayaraja has gone beyond them effortlessly. Reportedly Rahman's music has definitely shown age, because it doesn't readily strike the subconscious, which melody almost mathematically, regularly does. Rahman’s music is being 'trendy', and 'youthful' and 'fresh' are many times just used to cover up for deficiencies.
Apparently it would be wrong to compare Rahman's workmanlike painstaking creations to that of an absolute genius and rahman is nowhere near Illayaraja when it comes to performing, or conducting live. Further towards this end, the biggest insult we can dish out to rahman is by referring to his Oscars, but Illayaraja needs no such validation. The sheer number of movie careers his music has launched by creating super hits has kept a whole industry alive. Hence Illayaraja is like none other in the whole world and how many composers can even come near 1000 feature films, two symphonies thrown in like a weekend picnic? So nobody gets there without being an outright awesome composer.