The quality of pictures

A corollary of Mycroft Holmes' theorem: the quality of pictures taken with a lens is proportional to your degree of confidence in the lens itself.
In other words, if you don't like a lens, you will not like the pictures it takes.

I instinctively liked the Tamron 18-270, but after a very deep comparison with the Sigma 18-250, I ended buying the Sigma, which is excellent (truth is, probably they are very similar), but every time I use it, I have a vague feeling that "it's not what I really wished", and I tend to underestimate the pictures it takes.

I'd say the same reasoning applies to the Sony 70-300G vs the Tamron 70-300: if you pay ~3x the price for ~1.2x the quality, it may be that you are buying some extra "confidence" in the lens.
It's easier to trust a lens that is more expensive.