The choice between real down and a synthetic filling looks at first like a price choice. A synthetic duvet is cheaper to buy. But the picture changes when you look across the years.
Lifespan
A real down duvet lasts ten years, often longer. It can be washed, refilled, made anew. A synthetic duvet usually lasts three to five years. It cannot be refilled. When the filling clumps or goes flat, the duvet is written off.
At night
Beyond that, they differ fundamentally in how they work at night. Real down absorbs moisture. Synthetic does not. Real down is light. Synthetic is usually heavier at the same warmth. Real down breathes. Synthetic seals off.
The reckoning
When you reckon over the lifespan, a natural down duvet is often cheaper than a synthetic alternative. And that is before we speak of what you feel every night.