Good choices ... camera man!
Just be careful with the 28-75mm ... I have had to return the ones I had to TAMRON for front focus issues. They were fixed (uh, "adjusted") and work great :D, but it did take some time. The warranty covers that kind of repair ... you just need to get it to them.
The 28-75mm f/2.8 is a Di lens (easily identified by the "gold lettering" on the black band around the middle of the lens) and was designed for Full Frame use, so on the α900 ... it would be totally transferable and excellent.
The 17-50mm f/2.8 is a Di-II lens (easily identified by the "black lettering" on the gold band around the middle of the lens)
and would only be able to be used in CROP mode, on the α900. Both lenses are great. Obviously, the 28-75mm meets up with the 70-200mm or 70-300mm better, if that is a consideration, but the 17-50mm offers a wider frame, at the other end, except, of course, when mounted on the α900 (in CROP mode), where they would be the same.