The 400 II is best...if you need a 400mm lens. If you need a 300mm lens, the 300 II is best. If you need a 50mm lens, they both suck.
neuro's in top form lately!
Q8-MC - Your question makes no sense. There is 100mm difference, and that grows when you start to use TCs. What do you want to shoot, why and what is your budget?