I'm looking for the best lens to photograph butterflies and birds in flight.
CPS (Canon Professional Service) loaned me the 300mm f/2.8 II lens and I like it a lot. It's like a smaller version of my favorite bird lens, the 600mm f/4 II.

However, the 300 f/4 L lens is almost half the weight, the MFD (minimal focus distance) is 1.5-feet closer and it is about 5 thousand dollars cheaper.
The MFD is important when photographing butterflies, but the f/2.8 II lens gets me to within 6.5-feet which is pretty good. Plus I could add an extension tube to get me even closer.

The MTF charts and user experience verify that the f/2.8 II has outstanding optical qualities; as good as it gets. Still, the f/4 lens is an "L" lens and reports and MTF chart look very good.

For a variety of reasons the f/2.8 II is much better with teleconverters but I'm not sure how often I'd be using them on this lens. Likewise the better weather sealing of the f/2.8 II is not a big deal to me.

One thing that I've not found in reviews is focusing acquisition speed. Would there be a difference? That would matter on moving wildlife.
The other thing that the f/4 was introduced in 1997 and the f/2.8 II was introduced in 2011. The newer design might have components could impact performance. For example, the chip in the lens might work better with newer cameras.

Thanks for sharing any thoughts you might have on the topic.