Depends on the genre, i feel sonu sir and arijit are both equally talented singers when it comes to vocal talent,so coming to genres

1. Sad song: Arijit is bit better, he has a heavy voice which reflects pain
2. Love song: Sonu sir is bit better, arijit’s love songs have pain in his voice like listen tum hi ho,raaz aankhein teri, while sonu’s romantic songs have happiness ,listen mai agar kahoon, arijit has sung many happy kind love songs like sukoon mila,itni si baat hai,love me thoda aur, but in that genre still sonu sir is bit better
3. Ghazal: Arijit is bit better, he has a very heavy voice which reflects pain , and his ghazals are just simply mind blowing listen, muskurahat,rishto ke manzhar,aayat
4. Bhajan: sonu sir is better
5. Classicals/semi classicals: both are equally good, Listen to sonu sir album of classically mild, especially soona soona , while arijit’s classicals and semi classicals are just lit, listen mast magan,channa mereya,phir le aaya dil,sajde,tose naina,naina etc.
6. Rock/metal : Arijit is way better, he has a heavy voice which suits rock songs listen milne hai mujhse aayi,bekhayali reprise,khawayishein,bulleya reprise or metal like hungama ho gaya
7. Sufi: Arijit is bit better, he has a heavy voice which suits in harmony and alaaps, listen sajde(it’s a sufi rock song),laal ishq

Both are great singers, it depends on the genre which one will i like more