You may be able to find a copy of the Tamron cheaper and if you do then you may want to get it as it is supposed to be as good as the Minolta. I personally would go for the Minolta over the Tamron at any price. Focusing speed wise they are both about the same. They are very good for portrait work but they are really designed as close up lenses. The Minolta can get as close as 1:1, is very sharp and does great close-ups. however it is not a speed demon, as it's design precludes that (it has very close gearing to ensure accuracy). As a general purpose lens with 100mm focal length it is fast enough for most types of photography but certainly not birds in flight. If you want the best in portraiture lenses, the Minolta 100mm is second to the Minolta 85mm f1.4 G but at $1000 +/_that lens is for the pros or well heeled.
I have both the Sony version of the 100 macro (optically the same as the Minolta), and the Minolta 85 f/1.4 G RS (there were several variations on this lens, with the 85 f/1.4 G D Limited being the most sought after - the price for a good copy of that makes the other versions look cheap!).
I tend to to use the 100 Macro much for portraits, and keep it just for macro, as I have the Tamron 70-200 f/2.8 USD which is much more convenient as a zoom, and has fast and silent AF.
The 85 f/1.4 is, however, a step up from either - a great focal length on FF, with lovely rendering of the image - from very sharp on the point of focus, to silky smooth OOF areas. It was expensive (~ £600, s/h, from memory, I've had it a while), but worth it!
A900, Minolta 85 f/1.4 G RS, 1/125 f/1.4