Against the standart marine, it makes no difference if you go powerweapon or powerfist wound wise, but you will strike before them with the pw and after them with the fist.
But as Darkblade said, the fist makes it possible for you destroy walkers and easier wounding for MC. At the same time, It actually makes it possible to take down a LR or a monolith. With FC the fist is going to have a terrible high str of 9, make it possible to penetrate even the thickest armour;)
I would go for the fist, and let the ordinary attacks take down marines and stuff like that;)
"Baby, Im a heretic, and your a spineless imperalist"