Canon 100-400 push pull IS L lens from 1998 compared to the 100-400 twist zoom lens from 2014.

[RayboyUSMC]

Is the performance of the newer lens worth the extra dollars? My shooting is mostly bird life to include my favorite the Humming Bird so I use the 10 FPS shutter and usually CA. I have the older lens and it is pretty good, but if there is better and not that much more, then I would like to upgrade.