I've been thinking about the same thing. Research tells me that the biggest difference will be in focus speed. If you're not shooting action you'll be fine. If you ARE shooting action it'll be frustrating. I can't remember which of the two had better IQ. None of this is from personal experience though so it's not really what you asked for, but hey... it's something for you to chew on till then. I also wonder if the Sigma zoom is opposite from Nikon's like it is on their 17-50 (or whatever the range is). That's not a deal breaker for me, but it can be a deciding factor.

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Throw a little light. If it doesn't come back, it probably fell off...
Cheers
John
Flickr
Nikon D80 (nicknamed Cylopsoid)
Tamron 17-50mm f2.8 Lovin' it. I could glue it to my camera and be happy.
Tamron 28-75mm f2.8 It's the older model w/o the AF motor and it focuses FAST. It's just like the 17-50, only longer.
Nikkor 50mm f1.8d <------creammmmmy
Nikkor 18-55 kit Now in a deep slumber.
Quantaray 28-82 Somewhere in the house.
Nikon N65 Back for the resurrection tour! Coming to defend the faith...
(This spot being saved for hopefully a G9 or G11) Shhhh, I know those are Canons...