Sigma 18-250mm vs Canon 70-300mm

I currently own the Canon XTi w/ Canon 17-85mm lens and I am looking for some more zoom to capture candid shots because the 85mm is kind of lacking.

If I get the Sigma I get a lot of convenience because I don't have to switch lenses and possibly miss the moment. I do however have a lot of overlap with my current lens. This is a pretty new lens.

If I get the Canon I get a little more zoom and maybe better image quality because the lens does not have to cover so much zoom. I do run the risk of missing the photo because I have to swap lenses. This is a pretty old lens.

I am not a pro, just like to take family photos and general stuff. I usually upload to my web site or print 4x6 prints and maybe the occasional 8x10.

So with all that said...what do you think?

Mitch