Sigma 55-200mm or Tamron 55-200mm

I am about to purchase a EOS 400D w/18-55 II Kit. As a second lens I am looking to but either of these two lens:

Sigma 55-200mm f/4.5-5.6 DC
Reviewed at: http://www.slrgear.com/reviews/showproduct.php/product/177/cat/31
http://www.ephotozine.com/article/Sigma-55-200-f4-56-DC

OR
Tamron 55-200mm f/4-5.6 Di II LD Macro AF
Reviewed at:
http://www.slrgear.com/reviews/showproduct.php/product/333/cat/all
http://www.ephotozine.com/article/Tamron-55-200mm-f4-56-LD-Macro-Di-ll

I had a preference leading towards the Tamron, until I read it had a plastic mount.

Has anyone used these lenses or can you make a recommendation?
Thanks!