Motion is an excellent complement to After Effects. They both have advantages and disadvantages compared to each other and depending on what you need to do. They even play together nicely (a Motion 2 project can be imported into AE and continue to be changed in Motion as necessary).
In answer to your question, this would be easy to do in Motion. You can create the falling diamonds using a particle emitter with a line source across the top of the frame, and the build-up of the diamonds can be achieved with animated Replicators or simple layering.
One of the advantages of Motion over AE is the realtime performance of the particle systems, and AE doesn't have something as simple as the Replicator to create patterns quickly and easily.