Both are excellent lenses. The thing to note is the recent different philosophies of the two companies. Sigma tends to maximize sharpness, sort of quality above all, so their new Art/Sports lenses are often big and heavy. Tamron lenses are usually smaller and lighter, and IMO better ergonomically-designed. In the lens you specified, the Sigma Sports version weighs 63.7 oz. vs. the Tamon which weighs 52.4 oz. That's a difference of 11 oz. The Sigma is a little longer, by 1/2 inch.

If I were to make a choice, I would choose the Tamron, even though it might well be a tad less sharp, jsut because of the lighter weight and in my book better ergonomics. Not bashing Sigma at all, I own their 24mm/1.4 and 50mm/1.4 Art lenses.