Should you take warfarin or aspirin, if you have had a previous stroke?

Yes. If you have had a cerebral embolism, you should be taking warfarin. If you have had an ischemic stroke, then you need an antiplatelet drug, of which Aspirin is the best initial choice and should be continued until a stroke recurrence, if any, develops. Then Aspirin could be replaced or supplemented by another antiplatelet drug.
Depends. Depends on what type of stroke you had. If it was ischemic(form hardening of the arteries) then Aspirin is generally recomended. If it is embolic( from a clot traveling to the brain, sy form atrial fibrillation), then warfarin is recomended.