Character attack and elemental attack bonus are separate terms in the damage formula, so they're actually completely independent of each other. What this means is that the relative gain from character attack depends only on the existing character attack bonus, and elemental attack on existing elemental attack bonus. Elemental attack is generally superior simply because character attack is available from more sources right now. At low character attack, elemental attack will still be better--simply because buffing from 0 is more effective than buffing a low number that's greater than 0.