EF 100-400mm f/4.5-5.6L IS II vs EF 300mm f/4L IS + Ext. 1.4x III

Daniel L wrote:

Nicer image? Hmmm. Well, i don't have answer that but in term of technicality - Color, contrast, sharpness, focus speed and image stabilizer....etc The new 100-400 II put a whooping on a bare 300 F4 IS. Forget the 1.4x II, there's no comparison.

SnappyChappie wrote:

A year or so ago i wanted a telephoto lens for my EOS 650D, i looked at a number of lens [virtually] around the 300 to 400mm focal length range. I was very interested in the old EF 100-400 f/4.5-5.6L IS (mk1) lens but i didn't like the push-pull zoom function, the EF 400mm f/5.6L USM was a good length - but no IS which i thought was essential for hand-held tele shoots! Thus i disregarded that lens.

So I settled on the EF300mm f/4L IS USM and a 1.4x III extender to increase the FL to 420mm at f/5.6, so a bit longer than the EF400mm at the same aperture but if i don't need that much tele i can just keep it at 300mil @ f/4. I must say I really like the images from the ol' clunky EF 300mm f/4L Expensive though (as they all are!)...

Now, recently, Canon released a new 100-400 lens, the EF 100-400 f/4.5-5.6L IS II USM which looks really good - to have the option to zoom out to 100mil would be useful for sure. So although i'm not in the market for a new tele lens right now i'm still curious as to which lens in your opinions' gives nicer images at their longest length - the EF100-400L II zoom @ 400mm f/5.6 or the EF 300mm IS L with 1.4 extender, so 420mm @ f/5.6? This is a subjective question of course.

So is this one of those rare cases when a zoom lens has better IQ than a Prime?    I must say i am quite interested in this lens even more now...