Simon,
"Pin cushion" or "barrel distortion" and "wide angle" distortion are two separate things.
The first is a fault in the lens' projection geometry, the other is the result of the angle of view.
While you can get rid of the first, the latter is impossible to avoid with lenses that having large angle of view.
But yes, i agree that in this case the lowest distortion would be preferable over the highest resolution, since adding extra distortion certainly doesn't help, and the resolution of the 38 mm lens is more than high enough already (you rarely ever use the high resolution of these lenses fully anyway).
But don't expect the Biogon to be less "sensitive" to wide angle distortion.
The optics of the Biogon remained unchanged from 1954 until the 905 appeared a few years ago with a redesigned Biogon.
The redesign was made necessary because of environmental restrictions on what glasses could be used.
The redesigned Biogon on the 905 differs in performance from the older lens, but only a tiny bit (which is quite remarkable, since the original design was made in the pre-computer era, while they will have used brute force computing to design the new lens. Shows what a good job the original design is). The resolution in the center is slightly less, while that in the corners was improved. Overall, i think, not a difference to worry about.
So all the versions since 1954 are equal as far as resolution and distortion is concerned.
And all the T* versions are the same as far as coating are concerned too, no matter whether C, CF, or CFi.
The CFi version promises to have better internal baffling, but i doubt that this is very apparent in the Biogon.