The Tamron is the
SP AF 17-50mm F2.8 XR LD IF Di II. I own it and love it - and there'd be bloodshed if you tried to take it from me.
When I was shopping for this range I really only considered the DA*16-50 and the Tam17-50. What won me over to Tamron was the price difference. Nobody in town sells either lens so I had to do my comparisons online and I just could not see $300 difference in image quality between photos taken by the two.
The DA* does have the silent SDM AF motor and weather sealing but that wasn't a major point for me. The DA* likely may also have better coatings but I've seen more examples of CA and PF from 16-50 shots than I have with 17-50 which may or may not hold any credence; I haven't seen any in my shots though.
Now that I have the DA*50-135 I can appreciate the smooth quiet operation of SDM (very nice), but if there was another 50-135 lens on the market with comparable image quality for $300 less I'd own it. If either of those two are really important due to the environment you shoot in then by all means get the DA*16-50.
The Sigma 17-70 2.8-4.5 wasn't on my radar because I was looking for constant aperture and it's a variable aperture lens.
Check this thread for example shots from the Tamron...
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/39195-wanna-se...wide-open.html