So, I want to get the Canon 400 5.6.
Have owned the Sigma 150-600 C and while happy enough with the IQ a few things began to bug me so it is gone. The weight after carrying it around on a sling became difficult and would cause headaches. This was with a sling and none of the other carrying options were appealing. The lens began to creep and zoom locks were inadequate to stop it while walking around which turned it into a 7 pound, 15 inch long weight swinging around.
So anyway. The 400 5.6 looks very appealing on weight and ease of use. The 100-400 mkII is obviously also a very good option. However, it is twice the price and will be at 400mm 90% of the time and only used for birds. Been doing a lot of reading the the prime in instances like this is still a very good option.
The cost of the mkII would also likely rule out an 80D or 7DII upgrade as well in the short term.
Is the 400 still worth grabbing new? Keeping an eye on second hand getting impatient.