I recall reading there's a few bugs where goblins usually aren't wearing armor on their limbs. If this is still true the serrated disks would be better.
Whether it's a bug or not, I can't say, but invaders have a large number of unarmored or lightly armored companions, like mounts. Against these, for sheer, immediate lethality? Discs. Always discs. Anything that doesn't die immediately typically has amputated parts, and bleeds out shortly after. Glass traps way out front backed by steel or iron is good for keeping enemies from jamming your iron and steel traps - which you need for the goblins that ARE armored.
Now, if you've got the mechanisms, Few things compare well to 10x MENACING STEEL SPIKES in a lever-linked repeat. That area will basically cause things to disintegrate in small doses at random.
If you don't, remember that dodging can wear you out, discs get 3 attacks per disc on a trigger, and 10x discs in a trap. That's 30 dodge/block/parry/armor rolls that have to pass before a goblin can take a second step. A zone of these traps can cause targets to pass out from fatigue, and IIRC, passing out on any weapon trap means auto-hits from the trap. 30x glass hits is plenty lethal.
For glass traps, I wouldn't bother with spikes or screws or axe blades, or balls. I'd go straight for the discs.