[LDiCesare]

Tarrasques are mostly useful because of their poison (and lots of hit points too). Tartarians are worthwhile commanders if they are not feebleminded, and then it's better if you can equip them.
So if you face poison resistant troops, it's better to use tartarians. Long term, I'd also go Tartarians since you have the Chalice, but if you just want to deal massive damage to whoever attacked you, Tarrasques look great.