Originally posted by newarts The Photozone DA 55-300mm test caused a calamity when it was published. Owners expected the Pentax to blow the Tamron out of the water, but the MTF scores were almost even. One theory is that the Photozone tested a defective 55-300, and this is supported by the mention of slight decentering in the test report. The other explanation is that Photozone's MTF test doesn't account for contrast, and that's where the Pentax really shines. A PopPhoto review showed the 55-300 as excellent on their Subjective Quality Factor scale, Their scale uses a combination of MTF scores and perceived IQ to rate the lens. PopPhoto SQF results are much more in line with how most people judge the image quality of a lens. The MTF scores at Photozone are again causing a furor in the new DA*55mm test, because it shows the 55mm as having the same MTF scores as the FA 50mm. People who have owned both swear up and down that the 55mm is sharper at f1.4-f2.8. My opinion is that the the Photozone MTF tests are accurate, but they only measure lines of resolution, not contrast, and it's contrast that is the more important criterion for IQ.
To get back to the original question, I have both. I had the Tamron, used it for a year, then bought the Pentax and used it for the next two years. My testing showed the Pentax was slightly sharper and the deeply saturated colours made my photos look better. Recently I went back to the Tamron for a few months and sent the Pentax in for repairs after a nasty fall. Once I got the Pentax back, the Tamron went back into storage. The Tamron photos are good, a bit too much PF, but just not quite as punchy. I imagine that anyone good at post-processing could level the playing field. One area where the Tamron wins out is in AF. The Pentax hunts once in a while and simply will not find focus. Quick shift focus with this lens is a almost a necessity. I've taken many photos I'm proud of with the 70-300, but IME it's not as nice a lens as the 55-300. Since the OP was considering a 70-200 f2.8, I assume price is not an issue, in which case the Pentax is the better choice.
The Tamron pseudo-macro is a nice feature, but I would not trade it for the 55-69mm range. If I need macro I use a real macro lens or pop a Raynox 150 on the 55-300. The 55-300/Raynox combo allows you to increase magnification from 1:2, all the way to 1.4:1 just by zooming (set focus at infinity). You have to stop down a LOT for DOF though, so you need bright light or a flash.