I bought Kaco Mellow a few months back. That wrote so beautifully in my hands I felt I must try another Kaco model. So picked this one, Kaco Sky . It is not apparent from the pictures, but the grip section is softly 'triangular'. The nib in the Mellow and the Sky ought to be the same, but the latter has not felt as good as the one in Mellow.
The build quality of Sky feels really good.
May be it is the combination of the ink with the nib that changes things. So I've filled both the pens with the same ink now. Will be comparing them in the coming days. Let's see.