Mercury has different architecture. Is it better? Hard to say, users will tell. When building Mercury I was inspired by R Markdown YAML.

Voila is using tornado, and for each user it is creating a python kernel. User can interact with notebook code with widgets (ipywidgets) created inside the notebook. Voila can serve one notebook at the time.

Mercury is built on Django+Celery. Widgets are defined outside the notebook with YAML. All executions are processed in the background. There can be served several different notebooks in the same time. The notebooks gallery is built-in.

In Mercury, it will be easy to add more options for notebooks execution. I'm planning to add notebook scheduling and option to convert notebook into REST API.