Which 1.4x TC for 300/2.8 (1 Viewer)

I am going to buy a 1.4 teleconverter for my Canon 300/2.8 IS. The only in-depth comparison I have found on the web is a German one (on traumflieger.de). Their conclusion is, that the Kenko 1.4x DG4 MCX was better than the Canon MKII. Actually they did not test the MKIII but stated afterwards that it would beat the MKII.

Since the difference in price between the Kenko and the MKIII is quite significant, I would choose the Kenko. Anyone around who has experience with both the TCs?

I can't speak for the Kenko extender but my Canon 1.4 Mk2 extender works well with my Canon 300 F2.8 L IS Mk1. I believe the Mk3 is better but apparently it is very marginal as they use the same optical formula.
If you are looking at some more reach then the Canon 2 x Mk3 extender works remarkably well with this lens - attached is one of the very first images that I took with this combination. Note this is just the RAW file direct from the camera that has been JPEGed and scaled - no sharpening etc.

I always found the Canon 1.4x MkII to work very well on this lens - I also tried a Kenko Pro 1.4x (not the cheaper 1.4x DG4 MCX) and there was not a lot ion it but for me the Canon edged it.

Personally I would avoid using the Kenko on this (and other similar) lens. I have had few 1.4x tcs over the years and have tried the Kenko, Sigma and both mkII and mkIII Canons on the 300 f2.8. Both of the Canons performed very well but in side by side comparisons I could not see any advantage to the more expensive mkIII, everything I have ready suggests that the improvements of the mkIIIs only show on the newer (mkII) lenses. I found that the Sigma while sharp enough did not AF as well, however it does work superbly with Sigma lenses. The Kenko is fine optically and almost on a par with the Canons but I have seen it behave in very odd ways. On some big primes it has sent the IS crazy making it unusable. This problem is not related to a specific model of lens but to individual lenses (was fine on a friends 500 but not on mine). For me by the time you have payed out for a big expensive lens it is worth buying the tc that is designed to work with it.

I haven't had a Kenko but I did have a Canon Mk11, and now a Mk111 1.4TC on my Mk1 300f2.8.
I haven't noticed any real gain in the new MK111 so if budget is an issue I would suggest buying a Mk11 on the used market too. I have two Mk11 lenses and personally I 'm not sure I see a difference on them either probably because they were both excellent !
cheers Dave

when you have a toplens en a camera from canon it is always better to use the canon extenders .