I would second TTMartin's suggestion for those two Sigmas. I have the 17-50 and it's amazing. The 70-200 has been on the short list for my next lens. So is the 50-150, but it's only for crop bodies (but so's the 17-50!). The 50-150 might have better optics too.
That Tamron is tempting for the current price (with MIR) of $350. For that price it's almost a toy, and could be a good knock around. Yeah, the variable aperture is not all that great, but the image quality is pretty good if you use it within its limits, and the VC is supposed to work quite well.
One HUGE and potentially fatal problem exists with the Tamron 70-300 VC however! Apparently, if you comment about how much you like the lens on a thread, the lens will jump out of your hand or off of a desk and
commit suicide. No lie! Just ask.
Anyway, look at the third party lens forum for a bunch of recent threads on all these lenses!
Here are 17-50 samples - a shot of a plum tree on a windy day. 2 100% crops @ 1:1, one from the top left corner, and the other is where the focus point was, right of center. I can't say enough good things about this lens. These are directly from file - no PP at all.
Note focus point in window area
This is the top left branch - not even necessarily in the focus plane @ f2.8
This was the lower right focus point