Teleconverters 1.4XE and 2XE

[glaiben]

I just purchased a 250/4 FE for use on the X1D-ii via the XV adapter.  I'm considering a teleconverter to extend the reach a bit.  The 1.4XE is said to better optically than the 2XE (and the price difference is consistent).  Has anyone tried both of these converters and compared them (ideally on a 250/4 FE, but that may be a stretch)?  The 1.4XE would get me 350mm @ ƒ/5.6, and the 2XE would yield 500mm at ƒ/8.  However, if the 14XE is significantly better, then perhaps cropping may be the better approach.  Any thoughts or info would be beneficial.

Thanks...gregg