its subjective, if its death metal or metalcore i think a male vocalist is better...im not saying that female vocalists cant gore or whatever, but people will begin to realise that its more of a gimmick instead, everyone would just look at the band only cos of the vocalist, even if the band members are all talented musicians. Look at Arch Enemy.
For gothic metal, female vocalists are a better choice because of the melody in the music, and of course, male vocalists arent really a good idea for gothic metal, i mean not many people know that theres also a male vocalist in Lacuna Coil heh.
For other genres, i would prefer female vocalists because females bring out a better range and melody in the songs. And of course, if shes hot, its a selling point for the band.