Size
The Rottweiler is much larger: about 45 kg (99 lb) as an adult, against 25 kg (55 lb) for the Belgian Malinois.
Lifespan
In UK vet records the Belgian Malinois lives longer: a median of 12 years, against 10.6 for the Rottweiler.
Grooming
The Rottweiler needs more grooming, brushing a few times a week, where the Belgian Malinois needs a brush once a week.
Exercise
Plan for plenty of exercise with the Belgian Malinois, and a moderate amount with the Rottweiler.
Temperament
They differ most on affection (Belgian Malinois: likes its own space; Rottweiler: a lap or leaning dog) and on sensitivity (Belgian Malinois: needs a calm, predictable home; Rottweiler: hardy, takes a busy home in its stride).
Alike
Both are rated the same on loyalty: sticks close to its people.