Has anyone obtained both certifications and could explain the differences in complexity or topics between the assignment and essay of the Developer's certification and the assignment and essay of the Architect certification ?
The answer is simple - one is for developers, and the other for architects.
In the developer certification, you are given a specification, from which you are expected to develop a working solution in Java. You must then submit the code to an assessor who will validate that it meets the specification and works correctly.
In the architect certification, you have to take a multi-choice exam to demonstrate your overall knowledge, then you are given an assignment for which you have to architect an overall solution. This includes class diagrams, deployment diagrams, load balancing, etc. You will have to have a much bigger picture of how the entire company's infrastructure and software design works.