The 17-50 does a variety of multi-purpose focal lengths fairly well. The 11-16 only does wide angle, and it does it REALLY well. It's a matter of versatility vs quality. The 11-16 is optically superior, but it's also a one-trick pony. The 17-50 has a wider variety of uses, but doesn't do them as well. It's also worth noting that both lenses have different purposes. If you want a wide angle, get the Tokina. If you want a do-everything-lens, get the Tamron.