Sigma 18 - 300mm or Tamron 16 - 300mm ?
From what I've seen (including a couple of YouTube videos directly comparing them), the Sigma sounds like it has slightly better image quality then the Tamron, but the image stabilization and build quality is better on the Tamron. The Tamron is $200 more new, but I can get a used Tamron for a bit less than a new Sigma.
Normally, I'd heavily weigh the picture quality when making my choice, but considering post processing programs can fix chromatic aberration and barrel/pincushion distortion given lens correction files (which exist for both these lenses), I'm wondering if a slightly better picture quality matters.
The Sigma has a USB dock which allows updating the firmware and tweaking the focus, but I'm not sure how much of an advantage that is.
The Sigma dock is pretty useful. I had to make some significant focal adjustments to my 150-600 lens when I switched to the EOS 5d. The lens was really soft on the new body and had very inaccurate autofocus. The dock allowed me to fix these problems. The process of fixing it wasn’t fast, but it worked really well.