Hey Deborah, thanks for the comment. Yeah really great question. It’s funny. After Effects is only used for animation. In some ways it’s totally awesome and it allows you to create anything you can imagine. In other ways, it hasn’t evolved far enough. While I haven’t used Principle, I’ve seen some really promising work. I’ll be reaching out to the Principle crew and see if they can tell me what of the 12 Principles their tool supports. To answer your question now, though, I’d say it really comes down to what needs you have. If you need to create an interactive mockup, use a prototyping tool like Principle (though I’ve seen some really cool interactive motion work coming out from the other heavy hitters as well like Proto.io, InVision, etc). If you need to create a high fidelity proof of concept, I’d use AE. Nearly every ‘ui animation’ gif you see online has been made in AE.