Upload & Sell: Off
Considering all the factors such as MFD, weight, price, versatility etc, I would go with Ef500/4 II. I would trade a little smaller bird in the frame with more enjoyable to shoot, faster reaction to sudden BIF action, thus more keepers etc(due to its lighter weight). After 120K clicks on my 1d4, I found 700mm is perfectly good for Birds and you can push to 1000mm with good long lens technique. Since, you already have the fantastic 70-200/2.8 IS II, I will sell the 300/2.8 IS L, 5DII to upgrade to 5D3 and 1.4 III and 2XIIITC.