Neither is universally better - the right choice depends on the application. Alongside corrosion resistance and mechanical properties in the specific environment, you should weigh ease of fabrication, availability in convenient sizes, and the overall cost of ownership.
With such a wide range of stainless steels and nickel alloys available, several may suit a given application. Other considerations then include ease of inspection and replacement in service - which can justify over-specifying where access is difficult - and a proven operating track record.
Super duplex alloys score well on a balanced scorecard, combining high strength, corrosion resistance and competitive pricing against nickel alloys. They can be restricted at very low or very high temperatures, and selected nickel alloys can outperform them for corrosion resistance in particular environments.