Hi. My name is*****'m a camera repair technician with over 20 years in the business and I'll be glad to help you with your question today.
One thing to keep in mind is that they are boyh older lenses and repair parts are no longer available for the EF 400, not even lens mounts, which tend to go bad and with a 400mm lens, image stabilization is a real plus, even on a tripod. If you are using it "Hand Held", forget it. It's too heavy and you'll never get a clear shot.
To make a long story short, you would be much better off buying a lens with built-in image stabilization. The results are astounding! Canon has IS versions of both these lenses, so does Tamron and Tokina for better value.
The bot***** *****ne is, you should probably think teice before buying a long lens without image stabilization.