Both are currently on sale
I like them both for different purposes
[ my * 70-200 is in Japan awaiting a decision on repairing it ]
have you considered the use of this
HD Pentax-DA 1.4x AW AF Rear Converter
Pentax-DA 1.4x AW AF Rear Converter
Introduction
Pentax released the long-awaited 1.4x autofocus teleconverter in March, 2014. This handy accessory mounts between your existing DSLR lens and the camera mount to increase the effective focal length by 1.4x, giving you extra reach at the expense of one stop of light. For example, a 300mm F4 lens paired with the teleconverter would give you the same field of view and maximum aperture as a 420mm F5.6 lens. We feel that the teleconverter is an important addition to the Pentax lens lineup, which currently has a limited number of extreme telephoto lenses.
Read more at:
Pentax-DA 1.4x AW AF Rear Converter Review - Introduction | PentaxForums.com Reviews
to extend the length of the D FA * 70-200mm F2.8
I don't shoot FF so I can't help on how much vignetting there might be
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another suggestion, consider renting either or both lenses to try them your self
I have used LensRentals.com and liked their service