Size
The Rottweiler is a little larger on average: about 45 kg (99 lb) as an adult, against 35 kg (77 lb) for the Rhodesian Ridgeback.
Lifespan
In UK vet records the Rhodesian Ridgeback lives longer: a median of 12 years, against 10.6 for the Rottweiler.
Shedding
Their coats shed differently: the Rhodesian Ridgeback sheds through the year, the Rottweiler moults most as the seasons change.
Grooming
The Rottweiler needs more grooming, brushing a few times a week, where the Rhodesian Ridgeback needs a brush once a week.
Exercise
Plan for plenty of exercise with the Rhodesian Ridgeback, and a moderate amount with the Rottweiler.
Temperament
They differ most on affection (Rhodesian Ridgeback: affectionate on its own terms; Rottweiler: a lap or leaning dog) and on independence (Rhodesian Ridgeback: decides for itself; Rottweiler: cooperative, with its own ideas).
Alike
Both are rated the same on loyalty: sticks close to its people.