This is a truly complicated question. It is true that both of these complications will prove to be deadly. However, in cardiac death, all organ systems will seize to operate within minutes of total cardiac death. Unlike cardiac death, cranial death is a much slower, and in rare cases, reversible process. Upon being rendered brain dead (or in a coma), the brain will no longer function as it used to. However, the body can be kept alive artificially through life support systems. Without life support systems, it is an almost guaranteed death. I hope this helped !