The age old question: Tamron vs Sony. I'm upgrading from the powerzoom 15-50 kit lens Sony gives you on my a6400, and I need something with a larger maximum zoom range, hence the 17/16-70mm range. I mainly shoot sports, portrait, and automotive, with some video on the side. The main reason I am torn between the two is the varying price, with the Sony being almost 1/3 the price of the Tamron. If anyone has shot extensively with either lens and would be able to show some demo photos, please tag onto the thread!

The difference between the focal length you have and the focal length you want to buy is negligible. Rather than try to replace the 15-50, buy a lens that will augment it. 70mm is way too short for sports, even on an APS-C sensor. Or if you really want and all-in-one, look at the 18-135. At least you'll be getting closer to a proper sports focal length. 

Also, I am not aware of a 15-50. There's an 16-50 power zoom.

I did mean the 16-50. I have a 55-210mm lens that I use for sports, and I find that that is too big for some sports, especially indoor ones. 

I'd till shoot for the 18-105 or 18-135. Both are top drawer lenses. 

I had the 18-135 as it came with the body, but I replaced it with the 55-210. I noticed a lot of vignetting on the wide end and it was not quite enough reach for outdoor sports. What I'm mainly after is a replacement for the 16-50 that has a little extra reach but maintains sharpness at the wide end. I have a personal vendetta towards power zoom lenses, so I'm not even considering the 18-105. At some point I do intend to replace my 55-210 for a 70-350 G OSS.