I have the Canon 300 F2.8 L IS Mk1 (simply an excellent lens). In the case of the 1.4 extender - it makes little difference whether you use the Mk2 or Mk3, I found little if any difference on this lens though the Mk3 is better with newer lenses. With the 2 x extender I found the Mk3 to be markedly better on any lens that I can fit it to and one that it is not supposed to work with. Frankly my Canon 2 x Mk2 was a waste of money (luckily I got a good price for it) as I was better off using the 1.4 and cropping. With the 2 x Mk3 the IQ is much better (especially on the 300 F2.8 L IS). Canon do not claim any AF advantages on Mk1 Superteles but I can state from experience that the 2 x Mk3 does AF better on the 600 F4 L IS (1) and 300 F2.8 L IS (1), not a lot but definitely better.
With the 1.4, just get whichever you can find cheapest. With the 2 x get the Mk3 or don't bother - just my thoughts.