"The 400/2.8L IS MKI is a beast! the 300/2.8 MKII is feather weight
for sports you can't beat the 400 especially now with FF cameras the 300 is on the short side."
Hi Sports:
The 400mm f2.8 IS L is definitely THE beast, even in the much lighter V2 version.
But the 300mm f2.8 IS L is more flexible and much easier to use. It takes 1.4x and 2x TCs well, giving 300mm f2.8, 420mm f4 and 600mm f5.6 focal length coverage with very good quality - the ONLY lens in the Canon stable I use the 2x on. AF does slow down some with the 1.4x and significantly with the 2x. I use this lens for baseball when my primary coverage will be the pitcher, infield and 1st base action, including runs to 1st up the 1st base line, shooting from behind the plate and from the 1st base pit. I do like to shoot a little loose and crop, too. And its soooo much easier to sling around. I'm often sharing space with local newspaper photographers who need to grab some shots and go, and they use the same lens, pretty much the same way.
The 400mm is king in my baseball work when it comes to 2nd base, 3rd base and outfield action shooting from the pit. Best way to nail the rare outfield flying or rolling catch, for example. It overlaps with the 300mm for home plate action but for pitcher coverage I have to shoot behind and above home plate at a lot of fields, to catch the windup and follow-through. Too tight, otherwise. I shoot at f4 for better depth of field and sometimes put on the 1.4x if I expect a long drive into left or center field, or (in a pinch) to catch some outfield action from behind home plate.
It takes a lot more practice to master the 400mm, and heavier and more expensive support equipment, too (mono and tripods/gimbal heads, cases/packs, pain medication for back, etc.) than the 300mm. I sometimes shoot hand held in a pinch with the 300, never with the 400. To get a real sense of the difference, any potential buyer should put them side by side in their cases, then pick them up.
Jay