Tamron 17-50 F2.8 or sigma 18-50 F2.8 ?

Which is better?
They cost a similar amount, but which is better?
I have a Nikon D80 and use it to photograph family pictures , so will not be making them bigger than A4 size.

High recommended: Tamron AF 17-50mm f/2.8 SP XR Di II LD Aspherical [IF]. Because it clearly better Sigma AF 18-50mm f/2.8 DC EX macro. I used Sigma 18-50mm EX macro and changed to Tamron 17-50mm because Tamron clearly than Sigma. My body is Nikon D200. You can see more at www.photozone.de or www.fredmiranda.com.