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This is all very helpful. Based on all the information I've gotten here and at some other forums, I've decided to find something that can go wider than the 28-75mm. Jason, I am starting to understand the crop sensor and how that affects my lens selection (e.g. a 28mm is not actually 28mm). I've narrowed my lens options down to four and I'm still struggling to decide between them. I'll post the four I'm debating (from most expensive to least) between and see if you all can guide me a little further:
The 15-85 is, by all accounts, an excellent lens, although I haven't used one. You will be a little limited by the smaller apeture, but the IS will balance that out if your subject isn't moving too fast.
I have a 17-40, and like it a lot, but I'm shooting full frame, so its a completely different lens. You will be a bit limited by the f/4 apeture and you won't have IS to help out. 40mm on the long end is a bit short.
The sigma gets pretty mixed reviews, just like most sigmas before their most current generation of designs - they seemed to have a lot of sample variation. They recently re-did their QC process and the almost universally well reviewed 35/1.4 is the first result. The first zoom to come out of their new line is the 17-70 f/2.8-4 and they just announced a 18-35 f/1.8. I expect they will release a new version of the 17-50/2.8 in the next year or so. I had a 30/1.4 for a few years and was pretty happy with it. It was a stellar lens for catching toddlers in action, and there's a new version of that out as well. While I was happy with my one sigma, it was one of the more universally liked Sigmas of the previous generation (the 17-50 isn't) - I probably won't spend my money on any Sigmas that don't get A LOT of good reviews - currently I'm only considering the 35/1.4.
I owned the Tammy 17-50 non-VC until a couple months ago and it was my most used lens. There isn't a better zoom available for a crop unless you spend enough for a Canon 15-85 or 17-55.
To sum all that up - of the lenses you list, I'd go with the Tamron 17-50. BUT, I think you'd be best served by using what you have for a while and finding out what equipment your style of photography needs. The 50/1.8 will be stellar for pictures of the new baby; once you have a toddler, you'll need to think about something else to keep up (and zoom out at they run straight toward the camera . . .)