When should we choose dopamine over dobutamine in treating congestive cardiac failure?

Sometimes use both. Dopamine has more affect on blood pressure (raises it). Dobutamine increases heart (myocardial) contractility so it makes the heart muscle beat stronger and subsequently increases cardiac output. They are often used together for people with heart failure and low bp.
Low blood pressure. If the blood pressure is low in CHF, Dopamine is better at raising it.