Is the version III 1.4X TC worth it over the VII on a 500 f4L IS VI lens?

I have a version 1 Canon 500mm f4L IS lens and I currently have a VII 1.4X TC for it. Is it worth it to get the VII? Will I be able to tell the difference?

I have this lens and both the II and III teleconverters. I use it on a 7D II and I can't see any difference. I also don't notice any difference in focusing speed. The 2X III is another story. Quite a difference there compared to the older 2X.

I am with Larry on this one. I could find no difference between the Mk2 and 3 1.4 extenders - though there is a big difference (as Larry notes) with the 2 Mk3 x extender.
Unless you are planning on getting a 500 Mk2 and a 1DX Mk2 in the foreseeable future then I wouldn't worry about it as either will be very good.
The Mk3 is a little better built but I would only consider one if the price difference was minimal.

Agree as well (although I don't have the 500 v1).

Even on the newer 400 DO II, I could not tell any difference between the 1.4 vI (which is optically the same as vII but without weather sealing) and vIII in IQ or focus (although I ended up keeping the vIII just in case it is better in focusing in some situations).

The 2x extender is another matter. vIII is definitely better.