Well, my experience is mostly in After Effects, but I have dabbled a bit with Shake. As far as I can see, After Effects is more of an allrounder, with better ability to work with motion graphics, while Shake is a much more specialized compositing program. Again, I have very little experience with Shake, but the workflow appealed to me.
I have done a lot of compositing in After Effects, and it has so far covered all my needs. Just needs a bit of creativity to find the right solutions. It’s no problem tracking moving footage, and adding muzzle flair, clouds, replace skies, faces of actors, combine greenscreen shots with other footage, etc.
As for duplicating people, and making a realistic crowd, I have never looked into it. But it sounds to me like you could have a hard time doing your cop-trick with three actors in a moving shot. I am not saying it can’t be done, but it sounds to me like a big effort.
Oh, and don’t depend too much on my answers. Compositing is not my main profession, I have done it for some music videos and short films, but mostly just to give myself a challenge. My main line of work is cinematography.
John-Erling Holmenes Fredriksen
https://www.runaroundfilm.no/