In simple terms, both drugs decrease the "stickiness" of platelets and make the blood " thinner" and hence make it more difficult to from clots.
Both are used separately in even in combination in patients predisposed to clotting. eg patience who have has a heart attack or strokes or irregular rhythms of the heart.
Neither is "stronger" to use the term loosely than the other and both have minor side effects
The only advantage in giving clopidogrel is where the patient is allergic or hypersensitive to Asprin.