I realize I didn't actually answer the question.

defence or attack?

I throw down emblems on defense for heroes that occupy tank spots, or need to heal. I thrown them on offense for heroes that are nukers.

I do this, not so much because of research, testing, and spreadsheets on damage, but rather, because I believe you should focus on increasing a hero's strengths, not try to fix their weaknesses. If a hero is built for damage, boost it further. If a hero needs to play the tank, or support, boost the defense.

I've found this true in my professional life: get really good at my strengths, and let my weaknesses be weaknesses. Just my two cents.