The 28-75 vs 24-70 ... Fill the same space but cater to it diferently.
I have had 2 28-75 lenses and have the 24-70 ( Tamron ).
I actually still have one 28-75 as a backup ( it got dropped and the edge of the frame chipped off, although it still works perfectly even after it had 2 KG's with of camer and equipment behind it on the fall )
The Tamron 24-70 is much heavier and a lot bigger
The Tamron 28-75 is thus light and compact.
Image Quality:
I did loads of test, and then a whole lot more and the 24-70 had a definate clarity and better image quality over the 28-75, however this is only when I start to really start to zoom in on the photos in Lightroom or Photoshop/Gimp or whatever you use to edit.
Tamron 24-70 (4/5)
Tamron 28-75 (3/5)
Build Quality :
The 24-70 is a solid piece of equipment and you it really feels like it belongs on a high end camera. ( 5/5 )
The 28-75 feels like its a good quality, but can develop a little give and have a rattle or 2 when shaking. (3/5)
Low light :
The 28-75 struggles a little more than the 24-70 with this although no lens is perfect and lower the light, the lest the contrast the more then lens will struggle.
Tamron 24-70 (4/5)
Tamron 28-75 (3/5)
Value for Money :
The 2 lenses are both excellent value especially when compared anything else in the range. The 28-75 may not be the top tier , but the price really makes up for it since you can easily buy 2 of them and still have change for lunch when compared to the 24-70mm
Tamron 24-70 (4/5)
Tamron 28-75 (5/5)
PS: I consider the 24-70 to a sharper lens with overall better images quality.
This is all my opinion and may vary due to wind conditions, the tides and whether or not I left the seat up.