The "Which is better?" question is frequently being aimed at the Sigma 150-600mm f/5-6.3 DG OS HSM Contemporary Lens and the Tamron 150-600mm f/5-6.3 Di VC USD Lens, the first major entry into the 150-600mm lens category. These two lenses are direct competitors, sharing many features including USD/HSM AF, OS/VC, build quality and lightweight design. From the image quality perspective, here is the Sigma 150-600 Contemporary Lens vs. Tamron 150-600 VC Lens comparison.
At the wide end of the focal length range, the Sigma is sharper with a wide open aperture. The Tamron is 1/3 stop wider at some of the comparison focal lengths (200mm and 400mm) and to be fair, I am comparing those focal lengths at the widest equal aperture. At 200mm, these two lenses are very similar in sharpness wide open. At 300mm, I'll give the Sigma a slight advantage and at 400mm through 500mm, the slight advantage swings to the Tamron, though the Sigma's corners are better at 500mm. At 600mm, the Tamron has a very slight center-of-the-frame advantage and the Sigma has a larger corner-of-the-frame advantage.
Stopping down to f/8 reduces most of the sharpness advantages one lens has over the other. The Sigma has sharper corners at 150mm and 500mm, but the Tamron has sharper corners at 400mm. The Sigma is noticeably sharper at 600mm, especially in the mid and peripheral portions of the image circle.
The Tamron has slightly stronger pincushion distortion and has more noticeable CA. The Sigma has more vignetting with a wide open aperture, averaging roughly .5 stops of stronger corner shading over most of the focal length range except at the 600mm end where the the difference is only about .2 stops. Stopped down to f/8, the vignetting difference at the long end remains small, but the Tamron holds an edge in the wide end corners. Corner shading differences at f/11 are not going to be noticeable except perhaps in 300mm corners.
This image quality comparison does not place either lens with a clear lead and either lens can be justified, perhaps with decision emphasis being placed on the focal length expected to be most-valued. Here is a list showing additional differences between the Tamron and Sigma Contemporary versions of the 150-600mm lenses:
Which lens is better?
I don't think that there is a right or wrong answer here, but I lean slightly toward the Sigma, partially because these lenses are going to most frequently be bought for and used at the 600mm focal length and, at least at f/8, the Sigma holds the optical advantage at 600mm.
Get Your 150-600:
B&H has the Sigma 150-600mm f/5-6.3 DG OS HSM Contemporary Lens in stock.
B&H has the Tamron 150-600mm f/5-6.3 Di VC USD Lens in stock.