I've only been using it for a couple of weeks now, but I do have a Tamron 75-300 lens that I use with my A100. I don't have experience with the Sony version, but I can tell you that I like the Tamron.
Judging by the age of your original post, I'd say you've made your decision by now, but just for kicks I did a little experiment in my kitchen. I put the camera on a tripod, set it to full auto, and aimed for the little tiny coke bottle in the middle of the display.
Photo 1 is taken with the zoom pulled all the way back from around 5 feet away. To give some size perspective, the words "is it!" are the tail end of a Coke bumper sticker. Just to the left of center and in the background is a CD case with the words "Santa's Helpers":
<a href="http://www.flickr.com/photos/25165298@N02/2386803120/" title="zoom test by spookie5150, on Flickr"><img src="http://farm3.static.flickr.com/2408/2386803120_ed258e6ac4.jpg" width="500" height="334" alt="zoom test" /></a>
Photo 2 is taken from the same spot with the zoom at the full 300mm:
<a href="http://www.flickr.com/photos/25165298@N02/2386804256/" title="zoom test 2 by spookie5150, on Flickr"><img src="http://farm4.static.flickr.com/3087/2386804256_9d6a9067e0.jpg" width="500" height="334" alt="zoom test 2" /></a>
No special lighting and neither shot was tweaked at all. This should at least give you an idea of what it can do.
Sorry if this was too late to help. If nothing else, it was educational for me.:)