Onlays advantages over fillings are that they are stronger, more durable and more stain resistant. On average such restorations will not need replacing for 20 to 30 years. Frequently questions are asked by our clients what is the difference between a Crown and Inlay? The significant difference between the two is that while an Onlay covers the tooth cusp, the Inlay only fills parts of the cusps. Crowns cover the biting surface of the tooth completely, including the tooth structure above the gum line.