So a package arrived for me today... (thanks VaughnA!)
It seems I may have reunited a pair of fraternal twins that were separated at birth!
There are quite a few small differences between the two. The Ouantary is a 28-90mm lens with a 1:2.3 "Macro", while the Sigma is a 28-80mm lens with a 1:2 "Macro". The Quantaray goes to 11!
Now here is the really odd thing - notice how much further the front of the lens sticks out on the Quantaray, despite the fact that both lenses are set to infinity and 50mm. I can only guess it has something to do with the lens bieng factory modified to go to 90mm.
I just noticed this - the Sigma has 8 aperture blades, The Quantaray has 9. Maybe they are more like inbred cousins!
Overall I would say the Quantaray feels slightly better built, but I'm splitting hairs to say that. I do like the look and the feel of the zoom grip on the Sigma better, but that is a personal thing, not a quality issue. I'm not sure if I like the gold stripe on the focus ring of the Quantaray. It is just silkscreened on, and I don't think it adds much. Again, this is a personal taste issue.
The lens coatings seem to be very different, more so than I could seem to easily catch in a photograph.
Tomorrow I will take the two of them out for a comparison run!
Edit: I just noticed that, unlike most of Pentax's current offerings, these two $20 wonders are made in Japan. In your face Pentax!