Ah. I am just looking into my next lens as either a Tamron 17-50, a Tamron 28-75, or maybe even a Sigma 70-200. The Sigma costs much more money, but I already have the 18-70 focal lengths covered. Come to think of it, I don't think I'm gonna want a Tamron 28-75, but the Tamron is definitely on the table and maybe even the Sigma. Just contemplating if that extra $300 is really worth it. It would mean waiting a much longer time to purchase one, maybe even months. I'm in school now, and am too busy with extracurriculars to have a job, so that extra $300 is going to take a bit of time to make.
On the other hand, if the Tamron can produce images of very similar quality, then I probably don't need the Sigma. Those focal ranges are nice, but not worth the extra $300. Plus, I can sell my 18-70 if I get the 17-50, which means a little extra money for other stuff.