The A220 is more my thing, so I'd have to pick its interior as the better of the two. But I'm not going to argue that the Model 3's isn't influential.
In the Model 3, as with the rest of the car, you have a welter of ideas and a pioneering attitude toward the auto interior of the future.
With the A220, you have an entry-level rendering of the best interior in the business, without having to pay an E-Class or S-Class price.
Different strokes for different folks. In my experience, Model 3 fans and Tesla enthusiasts in general don't crave traditional luxury; rather, they want an interior that comes off as high-tech and minimalist.
But Mercedes customers are another story: They're paying for proper luxury. What's wonderful about the A220 is how accessible Mercedes has made that brand value.