I am very curious to know if anyone has had the opportunity to compare Sigma's 18-250mm walkabout lens against Tamron's 18-270mm?

I'm particularly interested in the comparison of focusing speed and accuracy between the two as I currently own the Sigma 18-250mm (which is an excellent all-in-one, by the way) but do face some issues with focusing especially in low light or when the lighting of subject blends in with the background.

Looking forward to your views.

Kind regards,