I absolutely agree with the first speaker. However, there is one more point: the heart can be transplanted or replaced with an artificial one. The brain, at least not yet. And, to be honest, I do not believe that it will ever be possible. The brain is our memory, our personality, our "I".
If the heart stops, then in the very near future the brain will begin to experience an acute lack of oxygen and glucose, since blood will not flow to it. The same is true for all other organs and tissues. Everything will start to "die", "die" .. This is a question for a couple of minutes.
Now about the brain. I will include both the brain and the spinal cord in my word, although perhaps this is not entirely correct. But the point is that all reflexes, all impulses come from him (both to the heart and to any other part of the body). Commands go from the brain, with what frequency and strength to beat the heart. And also from the brain there are commands with what activity the organs should work (we are talking about the autonomic nervous system, that is, about the unconscious). But. The heart has an automaton, pacemaker cells that can generate impulses that give heart contractions without external impulses. Therefore, when the brain is "turned off", the heart will be able to tap autonomously for some time, but again for a short time, its autonomous impulses will last for several minutes.
Thus, when the work of both the heart and the brain is "stopped" there will be a couple of minutes countdown. Answer: the brain and the heart are equally important)) without one the other cannot.