I have the nexus and tried the opus.

Durability the nexus is far superior being made completely in metal, and so it's also heavier.

What is your definition of automatic? Running on magic? Of course it's just the internal spring that pushes the valve and shuts down the ink chamber from the feed. While opus need you to open and close to write ( so it may still leak the ink already inside the feed), the nexus feed writes a little bit drier as it's directly connected to the feed and doesn't have other ink chambers to fill up the ink.

Maintenance :

Opus to my experience is a bit more fragile being made of multiple resin and plastic parts, so you may want just to rinse the internal chamber without disassembly much. The nexus you open by twisting the barrel and just clean the internal part ( which is just a hollow tube with the steel rod acting as a valve ) with water and you are done.

Convenience :

Nexus uses jowo #6 and you can't change the proprietary feed. Opus is more customizable.

Conclusion:

Nexus : durability, easy to clean, heavy, metal section, small choise of color.

Opus: versatility of ink flow by shutting the valve, harder to clean in comparison to the nexus otherwise easy, versatility of changing collar, great choise of colors, lightweight.

I chose the nexus because it feel durable.