Which is better, Ironmight shield or Grand Tortise?


Ageed, disregarding the minor slowness.

Ironmight and Grand Tortoise's normal defences are identical. The only difference between the two is that Ironmight has Pierce defence while Grand Tortoise has double damage on it's charge. I'd say Grand Tortoise wins 2/3 of most situations where there is a lack of piercing type enemies.
I vote for Grand Tortoise.

Then it's a matter of comparing movement speed+piercing defence to an epic shield bash. I'd totally go for the Tortoise if I just wanted something cool but in reality, if you really wanna get this shield used, Ironmight is probably better - or at least would be if piercing was a problem. In reality, the best choice is probably Volcanic Plate Shield. It has more applications thanks to the fire resistance.

Like I said before- the game is far more balanced than we credit it as being, and in this case, the balance is in the rest of your loadout.
If you're banking on MSI: Very High saving you while you strafe about with guns and bombs, then grand tortoise is a terrible idea. If you're using a sword and need to chase down enemies to smack the crap out of them, then grand tortoise is invaluable. Gremlins trying to give you the slip? Run after them, stun them with your huge area stun and murder them. These factors are far greater than whether you have piercing defense on your shield or not.

Swiftstrike buckler.
But in seriousness, the Ironmight is only best used vs giant lichen colonies (do pierc + stun), so as Krakob said, the Volcanic is most useful plate.
And as Fehzor said: it comes down to your own loadout/personal preference. If you want msi: plate. If you wana stun the #&@! out of stuff, get the torto shield(s).

I have a suspicion grand tortoise has ancient plate levels of normal Defence. The only way to make the defense stat bars not lie when comparing would be to have a piercing defense UV on the grand tortoise/ancient plate and then compare to ironmight.
This strategy also shows how different in normal defense ancient plate armor has compared to ironmight and volcanic plate.

Ironmight is the better choice since that Torto shield is just reused defense numbers from the Ancient Plate Shield. Ironmight becomes much more useful with a Max Status resist, like freeze for example.
No idea which has the most defence but they share health and stun resistance while Ironmight also comes with piercing defence and without the debuff. I'd say Ironmight but Grand Tortoise might have significantly better normal defence, I don't know. Don't forget about the shield bash, also. Grand Tortoise has a really cool one.