Sigma or Tamron lenses?

I can't seem to decide which brand I want next. I know Sigma has better build quality than Tamron but is a little more expensive. Tamron is cheaper but not as solidly build.
Which brand would you buy?

P.S. I'm debating on getting either the
Sigma 18-50mm/Tamron 17-50 (2.8)
OR
Sigma 24-70mm/Tamron 28-75mm (2.8)
for my Nikon D3000.
Any input on these lenses will be appreciated.

If I really had to choose, I would by the 17-50 mm (that little mm can make a big difference when shooting indoor architecture

My first choice though would be the Tokina AT-X 16-50 mm PRO The best build of the three.

Same with the 24-70 mm. The 28 mm is just a little too long in a lot of cases.

Too bad Tokina does not make a 24-70 mm f/2.8 yet

"I know Sigma has better build quality than Tamron but is a little more expensive. Tamron is cheaper but not as solidly build."

the above is all news to me,

i would get the 18-50mm myself

I own one Tamron lens that has issues so I don't use it… At least not very often. Because of this, I
don't recommend Tamron.