Hmm maybe you can take a look my recent create a website and showing my previous works..
Over there can find up the suitable distance of length for your right choice
VintagePhotography - Home
Hope my website can be help you something as what your shooting style..
Actually sigma had deliver a good IQ and lens body of built.
But the price has slightly higher than all.
For my previous tamron 17-50 VC also can produce a good photos and this one sigma maybe much better for you.
For the AF speed should be the same as SWM.
Thanks