It is really hard to make a direct comparison between Motion and AE. Besides both being Motion Graphics/Compositing applications, they really are quite different. So it depends what you mean when you say serious works.
Motion is an amazing real-time graphics engine that you can use to do a lot of things better/faster than AE. It's streamlined and built for doing certain things like animate titles and do simple compositions very well and very quickly. It is like a missile cruiser.
After Effects is more like an aircraft carrier. It can do the same things with more features and carry a lot more. The tradeoff is it is big, more complex and not as agile. So it takes a little more time/effort to to get the simple stuff out. However, when you need to create complex compositions that have dozens to hundreds of layers, lighting and effects, AE will keep on sailing long after Motion has started taking on water.
They are both very good applications suitable for professional level work. If you are asking about which one to learn, I'd say learn them both. The concepts are similar so it will be easier to learn one after the other and see where each has its advantages. If you are asking which one to buy, the value of Motion is hard to beat for $50. Depending on what you want to do though, AE might be the better tool for the job.