Tim,
I've used all three and I suspect Dave will chime in as well. The 1.7x was my number one choice due to balance of magnification and speed loss -- and it ended being a non-starter being basically pretty bad performance all around on this lens even stopped down. The 1.4x by contrast is excellent, showing almost no image degradation at 280mm, remaining very good even wide open. The 2xiii is marginally below the 1.4x, but still quite good even wide open at 400, clearly better one stop down though. I will gladly use the 1.4x anytime, and happily use the 2x when needed.
With regard to all three teleconverters use on the current 70-200 f2.8 VRII, Jack nailed it! I believe why the 1.7x has become sort of the orphaned child is simply this.... Prior to the current redesigned 2x TC-E III, the previous 2x's were marginal at best and often times unusable. That left only the 1.4x, generally outstanding for a 1.4x and the 1.7x being just OK at best and only when the stars aligned. When Nikon released the current updated 2x, its performance surpassed virtually all 35mm 2x's and their own 1.7x by a wide margin.
It's been rumored that the 1.7x may be updated with a aspherical design much like the current 2x, and if so, the natural order of expected performance might be restored 1.4x--> 1.7x--> 2x . Until that time, I would simply follow what Jack has concluded. Of course both shooting situations and prevailing conditions as well as lens selection when using the 2x, requires additional consideration and thought to get the most out of it. Camera to subject distance on certain lenses plays a substantial role as to just how well the current 2x does. Long mid-distance to infinity range is generally the weakest when attached to certain lenses.
*** Tim, Unless I'm mistaken, you won't be able to mount the current Nikon Af-S teleconverters to Zeiss lenses (nor any Nikon non Af-S lenses)...unless you make a simple external modification to each teleconverter. There is a square notch on one side of the teleconverter that prevents it from mounting and sitting flush with the lens its attached to, unless you dremel away that notch. It serves no function other than to prevent mounting to non Af-S lenses. Many have successfully done this modification, but of course it voids the Nikon warranty. If you decide to do it, watch for bits that are filed away as so that they don't enter the teleconverter or fall on glass surfaces.
Dave (D&A)