The difference in FOV is 'slight' but noticable. More on the apparent difference below.
First, lets consider EV. Not saying the add'l 18% tighter FOV is not worth it; it can be if you shoot under shade or canopy of trees as it's also even faster then the Sigma. Consider if you used the Sigma with an imaginary 1.2x TC to get it to 600mm, the Sigma + TC combo would be letting in the light of an f/5.4 lens vs. the f/4 of the 600mm Pentax; almost a full stop faster. And you know how much $ cost a full stop difference can make in ultra long lenses...
Otherwise, for the -best- illustration of the difference the FOV give you, try this out:
1) Load
Focal length comparison tool, Tamron USA
2) Click on their 200-500mm (I know, it's not a 600. Give me a second and hear me out. We only care about the FOV difference.)
3) Make sure digita is clicked (just so you can relate it to, and see, the
apparent FOV of the
actual lens on a 35mm camera ... just for illustration)
4) now use the lower slider and slide it so you see the "Angle of View" change between 5 and 4.1 degrees.
Now of course, if you're using both the 500 and 600 on digital, the apparent FOV is smaller on your digital camera. But the physical difference
in ratio does not change! Therefore the comparative difference in what this slider illustrates remains 100% valid.
Also, if it's an 8lb lens or a 12lb lens, you're using a Tripod, right? So the issue's just huck'n it. That difference in weight isn't unbearable, IMO at least.
Good luck if you bid.