Which of the following can be considered as the better restorative option in a grossly carious tooth?
Onlay restorations have many clinical applications and may be desired by many patients. These restorations have a well-deserved reputation for providing excellent service. The cast-metal onlay restoration spans the gap between the inlay, which is primarily an intracoronal restoration, and the full crown, which is a totally extracoronal restoration. 
The full onlay by definition caps all of the cusps of a posterior tooth and can be designed to help strengthen a tooth that has been weakened by caries or previous restorative experiences.
It can be designed to distribute occlusal loads over the tooth in a manner that greatly decreases the chance of future fracture.
It is more conservative of the tooth structure than the full crown preparation, and its supragingival margins, when possible, are less irritating to the gingiva. 

Sturdevant operative dentistry 7th Edition, Page No:e116
Cast metal onlay
Cast metal inlay
Full crown
Partial onlay