While the TC-17E improves when stopped down to f/8 effective, the TC-14E is already rather good wide open, almost on par with the TC-17E at f/8.
That's two stops difference which I personally value much more than the relative higher reach attainable with the TC-17E.
The TC-17E at 340mm wide open performs better (see next comparison) but stopped down to f/8 it's not better than @280 mm
at the same aperture, so I still prefer the images from the TC-14E which can be interpolated later in Photoshop to the same 340mm
magnification if needed, with comparable results.