Ok, I think I've gone through enough goblin volunteers on this fort.
Everything was done with masterwork mechanisms and exceptional/masterwork components (first two moods were mechanic and glass maker).
Spikes- Came out on top for killing without jamming, very little severing. Consistently gave fatal wounds, without actually killing, by the second round of attacks. Did a decent job against head and torso armor. Never jammed once.
Discs- Up there with spikes on lethality, gave fatal wounds without actually killing. More severing that spikes, but not nearly as much as metal discs do. Easily deflected by armor but does a good job of finding unarmored parts to slash.
Blades- Messy, messy, messy... Severed multiple limbs in each round of attacks, very prone to jamming. Turned 20 invaders into over 80 hauling jobs. Did good against metal torso armor, helms, and shields.
Screws- Extremely deadly, severs parts like it's going out of style. Good against armor too, I've got combat reports of screws shattering skulls and tearing apart brains through *iron helm*s. Tends to send heads flying, and jams a lot.
Balls- Didn't do much more than bruise, repeated applications lead to generally intact and well tenderized goblins. Slows them down though, seems only really useful to soften up invaders for your military.
Thanks to Torgan and Lemunde we've got two confirming data sets on axe blades and discs. Would anyone like to chime in with their own experiences, help confirm observations?