It should be the 75-200 F3.8.
As far as image quality goes...
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/60758-sigma-75...yone-know.html Right click and select Original for full size Duck | Flickr - Photo Sharing!
200mm F4 on k-7 at ISO400 (very noisy, but seems sharp at 200mm F4)
Tulips | Flickr - Photo Sharing!
200mm F5.6 on k-7 at ISO200 (looks sharp)
Cottage | Flickr - Photo Sharing!
105mm F5.6 on k-7 at ISO200
I would say the image quality looks to be pretty good (harder to judge because it is on the k-7.. and the images are noisy ;x)
For comparison, my Tamron 70-200 images:
http://farm9.staticflickr.com/8482/8274096584_91572204d6_o.jpg
122.5mm, ISO100, F3.2, Pentax k-x
http://farm9.staticflickr.com/8278/8759457127_3acb75bebb_o.jpg
200mm, ISO400, F4, Pentax k-x
http://farm9.staticflickr.com/8304/8012788258_de24229851_o.jpg
200mm, ISO100, F4, Pentax k-x
So if it's image quality, I would say the Tamron is better. But the Sigma 75-200 F3.8 is a very good lens from what I can see.
Also, have to consider,
manual focus vs autofocus, push-pull zoom vs twist-zoom, small vs huge and heavy, old vs new, etc.