I love my Tamron 70-200, though it was in to Tamron for
two months for repair of a sticky aperture mechanism. The six year warranty does instill some comfort
. I've compared the focusing speed of my Tamron vs. my DA* 50-135
here... I haven't handled the Sigma, but I hear good things about it (search for Jimbo's posts, he's doing great wildlife work with his...) If you can stand to loose a stop, I'd seriously consider the Sigma 100-300 f/4 also - I hear raves about it and you do already have the 50-135mm range covered .