Originally Posted by simonkit
(snip) basically a rebadged Tamron - no differences other than the SMC coating the advantages of which are probably pretty minor (snip)
The Tamron lens employs it's own proprietary coatings, including internal surface and multiple-layer coatings. While designed to do roughly the same thing as Pentax's Super-Multi-Coating (SMC) technology (increase light transmission, reduce glare, enhance image contrast, and minimize reflections), there is nothing available at the moment to suggest which might actually perform best in the real world. But, given that both lenses employ multiple coatings intended for the same things, I'd have to agree with your conjecture that any differences in this regard would likely be very minimal.
In fact, based on that notion, I purchased the Tamron lens instead of waiting for the Pentax version. While I cannot say how those two particular lenses compare, the Tamron does focus much quicker than the Sigma 18-200mm in both normal and weak lighting. For example, the Tamron lens quickly focused on a black chair in a poorly illuminated room, something the Sigma lens was not able to do at all. So, while I haven't tested the Tamron lens extensively yet (waiting on another order to test all at the same time), I do feel it is an improvement over my previous walkabout lens (the wife is now using the Sigma lens on her new K10D).
stewart