Polars has all of the benefits of DuckDB (to some degree), but also allows for larger-than-memory datasets.
To expand on this, Polars' `LazyFrame` implementation allows for simple addition of new backends like GPU, streaming, and now distributed computing (though it's currently locked to a vendor). The DuckDB codebase just doesn't have this flexibility, though there are ways to get it to run on GPU using external software.
Being able to use distributed backends to process frames sounds kind of incredible, but I can't imagine my little projects ever making use of it. Still, very cool stuff.