Assuming die area is the constraint, the choice is between 4 firestorm + 4 icestorm and 5 firestorm, because a firestorm core is 4x bigger than an icestorm core

Assuming die area is the constraint, the choice is between 4 firestorm + 4 icestorm and 5 firestorm, because a firestorm core is 4x bigger than an icestorm core. Or between 8 firestorm and 7 fire + 4 ice (or 6 fire + 8 ice).