Originally posted by Dan Rentea Canon 24-70mm f4L has 4 stops image stabilisation on the lens, it's weather and dust resistant and has a semi macro mode with a 0.7x magnification. Pentax however is one stop faster, but it's bigger and 187g heavier. I'm not sure wich is better optical because I haven't touched the Pentax lens.
I do hope Pentax full frame will be a superior camera to 5D Mark III and I do hope it's at least the same in terms of performance as Nikon D810 which is an impressive camera.
Considering the 24-70 f2.8 is the same as the Tamron 24-70, you can compare the two lenses on Photozone. Klaus there, really didn't like the 24-70 f4 due to some focus shifting issues, while he gave the Tamron 24-70 f2.8 a really good review.
I own the 24-70 and it is a really nice lens. I've mounted it on a film camera and it seems fine (doesn't auto focus of course), but I'm sure it will have more vignetting on full frame (in reviews seems to be primarily at f2.8 and 24mm).