If you're comparing the Sigma 150-500mm F5-6.3 APO DG OS to the Tamron SP 200-500MM F/5-6.3 Di LD (IF), the Sigma will probably win.
It has 21 elements in 15 groups verses Tamron's 13 elements in 10 groups, plus the Sigma has OS.
The Sigma also has more range, can focus closer and is bigger and heavier than the Tamron.
Sigma also has a 50-500mm f/4.5-6.3 APO DG OS HSM SLD, but that's in a different price range than the other 2 lenses, so a comparison wouldn't be fair.
If you're considering this one, the competition would be the Canon 100-400mm IS.