Lens Choice: 70-200 f/2.8 or 120-400 f4-5.6??

[NickN]

I am about to take the plunge and buy a new telephoto lens.

I have owned the 50-500 Sigma and found that the max aperture of f6.3 meant that when shooting at f8 I wasn't even a full stop down and the images were at times a little soft.

My thinking is with the 120-400 I will at least be a full stop down at f/8.

I have also seriously considered the Sigma 70-200 HSM f/2.8 but am just a little concerned about the short focal length.

Can people with experience with these lenses offer some feedback?

Keep in mind I shoot with Pentax so please don't recommend inapproriate lenses or start the old canon vs nikon vs rest of the world argument.