Ghost Weaver is intended as a builder spidertron, supposed to be used with bots to build stuff, one of the laser can heal / repair stuff (it doesn't have any auto aim for healing) while the other laser is to damage (it doesn't have that much damage though it does have unlimited ammo), in general it just has more storage compared to mk3.
Immolator was a test fire related spidertron, the idea was that it should have a passive ability and an active ability:
passive ability: it leaves a trail a flames wherever it goes ... and not ordinary flames but purifier ones, blue flames that deal a lot more damage than normal flames (you can disable this in the settings since in multiplayer games it can be really chaotic and creates a lot of pollution), you can easily kill bitters with this by just running away and letting them follow you and dying from the flames.
active ability: it unleashes a wave of purifier flames starting from your point, this ability will melt your biters away, at least when used in the proper situation (the buttons for it i think was shift-F, check options for mods it should show there)
It also flamethrowers similar to the tank but that fires further in comparison.
I guess the conclusion would be that the immolator is more for the pyromaniacs lovers out there while the ghost weaver is more for the builders out there that are not fans of fighting, on a side note these 2 spidertrons are more in between mk2 and mk3, so they are specifically made to be weaker than mk3.
Maybe some ppl have other opinions about the 2 spidertrons and to be honest i'm quite curios to hear about them.