Well, I think you need to learn both. But choose which one you need first. Maybe attack After Effects first as it is industry standard. Saying this Motion is pushing forward specifically in motion graphic field. I also believe that not only the program allows you to create amazing graphics. It is also important to say that a individual have a lot to do how creative something is. Please bear in mind that plugins are important but not crucial to decision making, let just say that almost all good plugins are now ported to motion.
Anyway I have seen Stephens work and it is amazing. He pushes the boundaries in using motion as a tool. Who is to say that you would not. Tool is just a tool, it is down to individual and their creativity to push the boundaries of creativity.
Maybe also see my https://www.embryo.me.uk website I make all my motion graphics with apple motion. (Please bear in mind I also know After Effects).