Originally posted by northcoastgreg 
The differences are subtle and, depending on the light, can take a sharp eye to notice. The DA* 300 is sharper (but you'll have to pixel peep to notice the difference) and just a bit more contrasty (although in real good light, it may not matter). The biggest difference may be color rendering. I would describe the Tamron color rendering as aesthetically distinct rather than 100% accurate. It's particularly good with warm and rich, dark colors: orange, yellow, dark blue, dark green. The Tamron is not so good (at least relative to the DA* 300) at light greens and light blues. There's also some subtle differences between how the two lenses render, although I'm not sure whether the differences here (mostly in favor of the DA* 300) are significant enough to matter in terms of practical output. I use both the Tamron 70-200 and the DA* 300 for volunteer work at the local zoo. I have noticed that the zoo tends to use more of my images from the DA* 300 than from the Tamron. Whether that's due to preferring the output of the Pentax lens I'm not sure. But the DA* 300 is the best lens I've ever used. It's about close to perfect as you're going to get at it's particular price point. The Tamron is the best bang for the buck lens I've ever used. It gets you very close to top of the line pro glass at a fraction of the cost.
Images from the DA* 300:
To give an idea of how sharp the DA* 300, consider this ~100% crop:
Here's a few more from the Tamron 70-200:
And a tight crop on the above image:
Some fine examples and I see what you mean, the Tamron does incredibly well and fights for every inch... the DA* 300mm is only just a tiny bit better to my eyes (for IQ)
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