EF 300mm f/4 L or f/4 L IS?

It's somewhat of a long shot, but has anyone here used both the original and IS versions of the EF 300mm f/4 L?  I'm wondering how they compare in terms of AF speed and sharpness. Does one take the 1.4x teleconverter better than the other? If IS and the closer focusing isn't needed, is the original one as good as or better than the IS version?

Both 300mm lenses are good. The non-IS is sharper, but you need higher shutter speeds all things being equal. Your mileage may vary hand held. 1.4x II slows down auto focus speed a bit. 2x III slows down auto focus a lot. Shot the Thunderbirds at the Pacific airshow hand held with non-IS and 2x III. Had to use 3200 ISO for adequate shutter speed. Many more AF fails than motion blur.

On an M50 (my camera with the smallest pixel pitch) the 300m non-IS with 1.4x II or 2x III a hair sharper than rezzing up. With 2x III, it's sharper than FD 500mm f/8 catadioptric, but mirror lens doesn't have chromatic aberration. FD 500mm is about as sharp as rezzing up without a converter. If you have something like a 6D or R6, you would probably never see the difference - the bigger pixels would never resolve the differences.

Castleman had a really practical review almost 20 years ago. The results still hold today and the differences probably widen with modern, higher resolution sensors.

Canon EF 300mm f/4 L IS USM Lens Review (wlcastleman.com)

Mind you his comments on the EF 100-400mm are about version I. Version II of the zoom is much improved.

Both 300mm's are relatively inexpensive and perform well. Non-IS is sharper, IS version lets you use a lower ISO.

User's choice.