I have the 24-105 and also have the Minolta 28-75/2.8 (supposed to be the same as the Tamron which should be easier to find at this point). I find the 28-75 to be an excellent lens on the A900. It's a bit bigger than the 24-105 but not much heavier. It's quite usable wide open and more uniformly sharp across focal lengths than the 24-105. The 24-105 is great at the wide end and even slightly sharper than the 28-75 at 28mm, but it seems pretty soft at the long end. The 28-75 is slightly sharper at 75 than 28, but the difference is small. I find the sharper, faster lens more generally useful for what I do than the greater zoom range. I'd love to have a Zeiss 24-70, but I think it would often stay at home because of how heavy it is. The 28-75 has been on the camera more than any other lens I have.
John