Sport or animal/birds?
400 2.8 IS is made strictly for sport shooter, weight as heavy as 600IS. It's not suitable for wildlife or small birds.
300 2.8 IS is used for sport as well as nature. For wildlife it's perfered for those like to hike, hunt and aproach with handheld or large mammals. Too short for small birds. It's light and very sharp with 1.4x, VERY usuable with 2x (notificable slower AF) . I have more keeper with 300IS+ 2x than 500IS + 1.4x for bird in flight.
It's a tough chioce when it come to 500/600 if money if not an issue
For small bird, one need 600 for reach. Though it's 100mm different in focus distance, image from 600 is about 40% larger than 500. But 600 is heavy, 3lb makes a lots of different when you have all the package together (tripod, head, flash bracket). You find youself lost interest pursuing subject in distance.
Also, the MFD is 15 feets on 500 compare to 18feets from 600. If you subject is at always at close distance, you'll have problem getting the shots. If travel on air, 500 is a better choice due to smaller size. If you do a lot of hiking, again, 500. If you do occasion handheld (3-5 minutes/rest interval), 500 wins. If you shooting from car, you better have larger front compartment for 600, it's not easy to move around due to physical length.
I read many advices and choices people make between these 2. Users that been there never regret picked 600 over 500. Those that have both use 600 about 70-90% more than 500. Only if they don't have to travel....
Since you ask, if you have 300, go for 600. for 70-200, may be the 500 will do.
Jim mentioned the FOV factors with lens. There are many users still perfering 1.6CF over FF for wildlife and sport due to image density.
Have fun, good hunting... join the club.

and we'll talk tripod/head
