I'd by 1.4TC and turn 400/2.8 into 600/4 when I need longer reach
My maths must be out as I get it to a 560mm not 600.
I'd go with the 400 2.8... simply because the sheer size of the 500 is too much for me (my arms aren't long enough to hook the lens hood!), plus the 400 is 2.8 and the 500 can never be. Sometimes its better to have f2.8 and use a crop tool than have f4 and get less accurate AF in failing light.