All I can say is that when these 'budget' super-zooms came onto the scene I did a lot research into three(four?) of them, Nikon, Tamron, and Sigma.
I took a lot of convincing and had the opportunity to get hold of the Tamron, having given up on the long-awaited NIkon. All in all I was phazed by the Quality Control of Sigma lenses (I'm sure it's improved but I'd been 'bitten' twice previously) so the Sigma went to third choice.
I found that Tamron wouldn't perform well beyond 550mm unless you stopped down to f/8 and I've heard that the same applies to the Sigma(s) - not doubt people will tell you otherwise. There are a number of tests/reviews out there which confirmed findings.
Why did I decide on the Nikon? Having a continuous f/5.6 throughout the zoom range is a big bonus and it does perform well at full aperture. And also,it does work with our Nikon1 Series cameras with phenomenal reach
Here is the
200-500mm thread ---- this is a very objective
comparison review with a bias toward birding