Someone hit a nerve
I see it like this, Giger is of course i brilliant artist, no one is doubting that, but he's just specialised to on single style, and that's the alien style.
Everything he did screams 'Alien', just look at Species, he's rehashing himself over and over again. Winston created characters that were different in character and style, he did a shitload of memorable characters, giving us the Terminator, Predator, Pumpkinhead, Jurassic Park... not to mention his kick ass practical effects work, that after 20 or more years look better than most of the stuff done today.
I do in fact think that Winston is the better artist.