If you were to look at the two you could see the difference. Build quality and image quality are VASTLY different. If you are going to get one, get the 70-300 Di.
There is NO comparison in quality.
I believe Sabbath is speaking from experience here. He is the reason I have recommended a few people to take the $135 plunge into a 6-year warranty lens (Tamron 70-300 Di).
Today, I finally got to use the Tamron for myself. WOW. For a $135 piece of equipment, this is no piece of junk. Even though I had high expectations based on Sabbath's previous reviews, this lens exceeded those. The build quality is far better (in my opinion) than the $160 Canon 75-300. The sharpness is also better as well, even wide open at both zoom extremes, sharpness isn't terrible (again, we're talking about a $135 lens).
Anyway, I now find myself debating if I should buy this lens for myself. I have a few thousand in camera gear, so it seems silly to add such a cheap piece of glass to it...but, a lens to take out to the lake on my boat? Yeah, we might have a winner. To sum it up, VERY impressed.
The sigma in these are the same price, are they better?
I don't have first-hand knowledge of this, but the Sigma doesn't have near as good reviews as the Tamron (Di version)
is the Tamron Autofocus 75-300mm f/4-5.6 LD better than my 70-300mm kit lens?
oh boy. Which brand 70-300 kit lens? (nikon, canon, tamron, what are the letters after it?) The Tamron
75-300 doesn't have good reviews (as Sabbath mentioned), so I see no reason why you should consider this over the Tamron 70-300