After Effects has been able to open layered psd files for a long time. It can even open layered files with live 3d layers on them.
Haven’t used Motion so I can’t tell you if it will be opening AE projects, seems unlikely.
Since Photoshop and After Effects are both Adobe, making them talk to each other is seamless, very easy. I don’t think that’s the case with Motion and Photoshop.
If your question is directed to which one you should buy, I’d say After Effects.