BIG caveat: after doing all this I realized that I need to create dwarves with two gauntlets and two boots, since asking for one item only gave you one of the pair. So keep that in mind. I'll try and revisit some of this a bit later to see if this matters.
Alright I ended up doing some more tests right away heh. Same idea as before (both sides use same weapons, different armor) except I used warhammers+shields instead of meteor hammers, used volcanic and mithril weapons, and tested steel armor as well.
Using mithril battle axes, volcanic vs mithril armor: volc 2, mithril 4 (this is same as my old one, getting a larger sample)
Using mithril battle axes, volcanic vs steel armor: volc 5, steel 1
Using volc battle axes, volcanic vs mithril armor:volc 5, mithril 1
Using mithril battle axes, steel vs mithril armor: mithril 4, steel 1
Using mithril pikes, steel vs mithril armor: mithril 4, steel 1
Using volcanic war hammers, volc vs. mithril armor:volc 1, mithril 5
So from all that, about the only thing you can conclude is that steel is indeed worse then both volc and mithril. Other then that, it does look like mithril is better, except for the results when volc battle axes were used.
Edit: the other clear thing is that there's something wrong either with meteor hammers or with wolfram as their material. The volcanic warhammers didn't have any of the same trouble penetrating mithril or volcanic armor.
Edit2: Instead of another reply I'll update this with what's probably my last test. This time I fixed the problem above with the armors, so every dwarf was fully armored instead of missing left gauntlets/boots. There are still unarmored body parts though in a breastplate+helm+gaunts+greaves+high boots setup, like the arms/face.
I made this simpler, just volcanic vs. mithril armor, using volcanic battle axes in one case and mithril battleaxes in the other. 11 trials.
With mithril battleaxes: 6 volcanic wins, 5 mithril wins
With volcanic battleaxes: 11 volcanic wins
It seems like with mithril battleaxes it's mostly luck based, basically who gets a cut on an unarmored body part first since neither one does much damage when hitting armor. With volcanic battleaxes, you can penetrate mithril armor easily, but not volcanic, hence volcanic always wins.
I think there may also be an effect where since mithril is so much lighter, mithril armored dwarves get more attacks, which would make sense of the advantage of mithril in the previous tests (where there were more unarmored body parts). Although this is much harder to tell, I did some 1 vs. 1 fights and looked over the logs, but it was ambiguous.
Final verdict (IMO): In FD mode, since no enemy uses volcanic weapons, mithril is probably better for armor, simply for the movement speed advantage and being relatively easier to make. Volcanic weapons are clearly better though.