It depends on the type of music. I play in a trio which does mainly vocal music with some instruments - 2 females and a male. Or in other words, soprano, alto, tenor. i think it's the perfect vocal combination because it allows for the widest range of possibilities without sounding top or bottom heavy. We're always careful to augment our vocal arrangments with an arrangment THAT FITS both harmonically and pitch-blend (overall voicing).

We can also do a wide range of repertoire & don't have to stick to single gender-specific songs.

I've just re-read your original thread, and realise you're going from a quartet to duo. I think duos can be great, but have always ended up singing in "third-type" harmonies, if you know what I mean. And the dynamic is different. But hey - the Louvin Brothers were just a duo, and so were the Everleys and they're both great (they are also sets of brothers!), and that's just scratching the surface!