I think it would be safe to say that a FMJ that does damage or rupture arteries or other vital organs necessary to maintain life in a human being, and that does exit would be a more lethal bullet.
On the flip side, I think it would also be safe to say that a JHP, in the grand scheme of things, is more lethal in that its expansion and total energy transfer inside the human body gives it a MUCH higher chance of causing damage to arteries or other vital organs (i.e. it expands and takes out the aortal wall when the FMJ does not).
It's a compromise. If the FMJ hits the right spots and exits, it's deadlier. If it misses where an expanded JHP would hit or doesn't expend enough of its energy to cause significant damage where an expanded JHP would, it's not deadlier. However, the chance of a JHP causing fatal damage and/or blood loss is more likely than a FMJ. In actuality they are two different arguments.