Given the reach comment posted by Sandy Hancock above... that would tip me towards the Sigma 70-200. If there is one thing that is disappointing about the 60-250, it's the reach. For a lot of my images, taken from 10-12 feet away, because of the internal focus, the DA* 60-25- is functionally about a 135mm lens. At infinity it should have more reach, but, I'd even question that. I tend to think of "more reach" in terms of a least 50% more reach. That's how much you need to be noticeable in every day use. 50 mm is only 25% more reach so half what I'd want to be considered to be a step up. Also the FL glass in my SIgma 8-16 gives amazing control of CA. I'm not sure pentax makes a lens that can match it. If the new SIgma has it, it could be a lens of the future. Meanwhile, Tess reported some purple fringing from the 60-250 yesterday, making me even more aware of this issue.
The DA* 60-250's control of CA is acceptable for 5 mu sensors (K-5- D800) only at 135mm. The Sigma AF 70-200mm f/2.8 EX OS HSM was worse than the 60-250, I've yet to see tests done on the APO 70-200mm F2.8 EX DG OS HSM, but if it's surpassed the DA*60-250's CA control, it could well be the lens of the future. But, buying it before someone has tested it for CA is probably a bit of a gamble. Looking at older tests, it had quite a bit of ground to make up before it reached 60-250 standards. The 60-250 @ 4 mu @135 mm is an exceptional lens and good enough for 24 MP APS-c sensors. I'd want to see numbers saying the SIgma is better before jumping in whole hog.
Quote: Of course as one of Sigma's flagship lenses, the 70-200mm F2.8 boasts a suitably complex optical design, featuring 18 elements in 15 groups, two of which are Extraordinary Low Dispersion (ELD) glass and two Special Low Dispersion (SLD) glass for correction of chromatic aberrations.
The thing with CA at the moment is, it's control is effectively judged by how many pixel widths it is. A lens that performs well on a 7 mu sensor may not perform nearly as good on a 5 mu sensor. WIth 24 MP APS-c sensors coming, my guess is that CA control is going to be a lot more critical. So, having the Sigma FL glass may be more important in the future than it is now.
Not only that but the Sigma 70-200 is said to be the fastest focusing option available on Pentax mount at the moment, at least in terms of long glass.
All those things are starting to add up. Buying today, I definitely go with the Sigma 70-200 over the DA* 60-250. For all the above reason, any one of which would be enough to swing me towards up. Taken together, it's a pretty convincing package.
But we won't know if it's the latest thing since sliced bread until we see some independent numbers verifying CA control. I own a 60-250 and I haven't seen enough from the SIgma yet to want to sell it. But, I' haven't been in the position Sandy Hancock is in, having had the chance to use them side by side, and that has to count for a lot more than my ramblings.