Having three Gets Hot rolls with only a 5+ invulnerable to save you is kind of rough.

I'm not a huge fan of either forgefiend, unless you need the long range anti-tank in a list full of other robots. Unless the rest of your army has tons of AV targets, they're too easy to prioritize and just burn away, and they're very expensive for the amount of damage they do in either configuration.

Now, the maulerfiend... it fills a completely different role from the forgefiend, but I think it actually does a decent job in that role, so it's worth looking into!