Size
The Greater Swiss Mountain Dog is much larger: about 51 kg (113 lb) as an adult, against 31 kg (68 lb) for the Labrador Retriever.
Grooming
The Greater Swiss Mountain Dog needs more grooming, brushing a few times a week, where the Labrador Retriever needs a brush once a week.
Exercise
Plan for plenty of exercise with the Labrador Retriever, and a moderate amount with the Greater Swiss Mountain Dog.
Temperament
They differ most on independence (Greater Swiss Mountain Dog: cooperative, with its own ideas; Labrador Retriever: keen to work with you) and on loyalty (Greater Swiss Mountain Dog: bonded to its family; Labrador Retriever: friendly with everyone).
Alike
Both are rated the same on affection: a lap or leaning dog.