The short answer is "both, and neither". For champs that are not DEF or HP, the stats are essential for survival, but beyond that, 100% useless.

If you're running an unkillable comp, you should ignore them entirely, even on DEF and HP champs, as cdmg is more useful.

For killable comps, you want to aim for 3200 DEF *after* auras and DEF up. You also generally want to be around 40k hp, since the a3 ignores DEF.