Sigma 18-50mm 2.8 or Canon 17-85mm USM?

jrnew wrote:

Go for something that will open up new possibilities for you. I have owned the 15-85, which I wanted for the wide range advantage over the kit, but other than that was nearly identical to the kit. I was disappointed and sold it because I didn't really use the wide end very much. I say this because I imagine that the 17-85 will not really change your current abilities and will even degrade your image quality in some regards if you're a pixel peeper (the 18-55 IS kit is amazingly competent in many ways).

I currently have the Sigma 17-50 and LOVE IT. Very highly recommended. So I can't speak for the Sigma 18-50, but assuming that it's constant 2.8 I'd recommend it because it will give you new possibilities. I rarely use my 50mm 1.8 anymore so I will likely get a longer prime lens in the future. In summary, I wouldn't change lenses just because you think the IQ will be that much better -- change lenses when you need more versatility. Go for a constant 2.8. And I wouldn't wait too long to get a flash (I love my Metz 50 AF-1, $199) so you can add even more versatility.