**Core Concept**
The main difference between composite and amalgam as restorative materials lies in their occlusal wear characteristics. Amalgam restorations exhibit higher occlusal wear compared to composite restorations, which is a critical factor in determining the longevity of dental restorations.

**Why the Correct Answer is Right**
Occlusal wear refers to the gradual loss of tooth structure due to the interaction between the restoration and the opposing teeth. Composite restorations, being more elastic and less prone to deformation, tend to exhibit lower occlusal wear compared to amalgam restorations. This is because amalgam is a hard, brittle material that can be more susceptible to wear and tear under occlusal forces. The difference in occlusal wear between composite and amalgam restorations is a significant factor in their clinical performance and longevity.

**Why Each Wrong Option is Incorrect**
**Option B:** While durability is an important consideration for restorative materials, it is not the primary difference between composite and amalgam restorations. Both materials have their own strengths and weaknesses in terms of durability.
**Option C:** Retention is an important factor in the success of a dental restoration, but it is not the main difference between composite and amalgam restorations. Both materials can be designed to achieve good retention.
**Option D:** Manipulation refers to the ease with which a material can be shaped and molded. While composite materials may be easier to manipulate than amalgam, this is not the primary difference between the two materials.

**Clinical Pearl / High-Yield Fact**
It is essential to consider the occlusal wear characteristics of restorative materials when selecting a material for a particular clinical scenario. Composite restorations are often preferred in cases where high occlusal wear is anticipated, such as in the posterior teeth or in patients with a high occlusal load.

**✓ Correct Answer: A. Occlusal wear**