Both, I suggest. If either is lacking too much, it will distruct, or bore, and detract from the performance.

A technically wonderful voice with no expression is an uncut diamond - lots of inherent value, but no beauty.

A poor voice but full of passion & empathy is a carefully crafted jewellery - out of paper. Pretty, but there's not many places you can wear it.

A good voice with good delivery - now, that's a jewel.

(And a poor voice that sings monotonously is Bob Dylan. OK, that's a tease...)