Quote: I think most people put the Signa 70-300 dead last among the major contenders (DA55-300, Tamron 70-300, DA50-200) when it comes to sharpness. If the 55-300 even used is too much, the Tamron 70-300 would be the next logical contender if you need 300mm, although you'll trade some CA for that extra sharpness.
Photo.do (?), for what it's worth, ranked the Sigma APO version as the only 70-300 budget zoom worth getting a couple years back. Go figure...
I found that if you bump the contrast way up, things look much better. I did side-by-side shots of the Sigma vs the Pentax 50-200 over the same focal length and the Pentax was only minutely better due to the snappier contrast.
Many people like the Tamron and it's certainly cheap enough. The Pentax 55-300 is way nicer than the Sigma 70-300 or Pentax 50-200, though the fringe is blue, not purple....
One aperture blade died on my Sigma, but I had already retired that, and the 50-200, after getting the Pentax 55-300.