INT is generally superior to MDmg in equal quantities, as dINT is
multiplied by some amount depending on its exact value and the spell in question. This value is usually >1, especially for the higher tier spells that a BLM favors. On weaker spells (T1/T2, helices) that multiplier may be 1 or even 0 at high dINT, but that's not worth building your weapon around. INT will also contribute a bit of magic accuracy.
INT doesn't add any damage to Death, but there's a cap on mdmg for that spell and you still get the extra macc.
That said, Keraunos is a bit dated nowadays. I'd worry less about getting a perfect augment than getting a newer weapon that will outclass it regardless.