At F4 there may not be too much difference.. the Real question is how do they perform at 2.8... well unfortunatly thats a game the 16-45 cant play (and it is a great lens I love mine)
Anecdotal evidence and biased owner here but I use the 16-45 lens by default and go to others for special purposes: DA 16-45mm Supporters Thread
Just some samples from a few days ago...
I have owned the 16-45 and the tamron 17-50 (still have that one).. The Tamron is the obvious choice for me, sharper (esp. corners) and less fringing in my experience.. I actually prefer it to the 16-50 (don't shoot me ;-)
I have the Sigma 17-70 too (my fiancee uses it) I would find it a bit more difficult choosing between this and the 16-45 (unless like for her the extra length is important), but it certainly doesn't win out over the Tamron 17-50 in my opinion...
As for the Pentax vs the Sigma, It's a simple matter of the sharpness. I found the Pentax to be a sharper lens, Even at f4 vs f4 for both. The Pentax SMC coatings render a warmer image with better flare control than the other brands but that's been a long standing advantage of SMC.
My impression, from memory, was that the Sigma fell between the Kit lens and the DA16-45mm and I was hoping for better and closer to the DA*16-50 but there's no free lunch. Of course that's just one lens and there can be sample variations.
Clearly from the comments above. The 16-45mm is a solid lens but user experiences with all 3 are variable.
You should have at least had the DA* 16-50 f2.8 in the list. The DA 16-45 f4 is a rather old lens design, which I think has been replaced with the newer DA 17-70 f4 SDM.