K Sanu dada has some problem with diction as well listen pardesi pardesi jana nahi, landing note me hi diction problem lagta hai. Plus he is nasal, copies Kishore hence Abhijeet, Shaan, KK and others rose in front of him. But K Sanu was a favourite of Nadeem Shravan they nearly gave all their work to him.
Yeah read this article "Udit and Sonu fight to sing a song in DON film, Udit wins"
http://celebworld.wordpress.com/2006/05/08/sonu-nigam-and-udit-narayan-fight-to-sing-khaike-paan-banaraswala-for-farhan-akhtars-don-udit-wins/
This proves that Udit's voice dominates Sonu's voice.
Who is a better singer? I think Udit because he is more experienced.