I moved your post to the DSLR forum because it wasn't a good fit in the thread where you posted it. Your question is a little vague. Both Sigma and Tamron make great lenses and mediocre lenses. It really depends on the specific lens you're looking at and what your budget is. If you tell us more about what you're looking for we can definitely give you some direction.
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I have lenses from many manufacturers, including Nikon, Sigma and Tamron. In my opinion you get what you pay for. All the manufacturers are equally smart at making lenses and just as competitive price-wise.
I tend to trust Tamron a bit more than Sigma. Sigma lens use a unique method of engineering their AF and Aperture control to fit the various mounts. As a result, frequently as camera bodies are upgraded Sigma lens lose this electronic control (rendering AF and or aperture control null). It can sometimes be fixed by sending the lens into sigma to be 'rechipped', which is still a hassle. I find that most of the time, whatever option Sigma has Tamron has a better, more fairly priced option. But as others have said, there is certainly some variance within different models and sometimes, depending on price, Sigma may give oyu a better option. If you post specific lens for us to compare, we can give you a more specific answer.
As was said, Tokina is also worth considering. Though they don't frequently have the same high performance optics as the competition, they are of a typically excellent build quality and very inexpensive as well. Their 80-200 does not compare to the Sigma or Tamron versions in optical performance, but their 80-200 is also one half to one third the price of the competition as well.
Feel free to edit and repost my work as a part of your critique.
I think you just need to read reviews and a lot of them. With either brand you can have problems with CA. I wouldn't buy anything with less the 41/2 stars. I don't buy a lens without researching. Most often just post a question here and someone has used the lens you ask about. The knowledge base here is deep.
I am like Barney Fife, I have a gun but Andy makes me keep the bullet in my pocket..
Question. Which lens for a DSLR camera is better. Tamron or Sigma?
Thanks for the objective answers I received from many of you. It is kind of difficult to decide which lens to purchase considering that some of these lens' prices are quite steep. Nevertheless, I can base my choice with some considerations to the answers given.