Technically life orb increases the damage of an attacking move by 30% not a stat. This means you'd have to use the increased damage and low atk vs the low damage and high atk and that's just a lot of math that smogon already solved for us.
252 SpA Life Orb Alakazam Psychic vs. 0 HP / 0 SpD Sunkern: 484-569 (240.7 - 283%) -- guaranteed OHKO
252 SpA Mega Alakazam Psychic vs. 0 HP / 0 SpD Sunkern: 451-532 (224.3 - 264.6%) -- guaranteed OHKO
So yeah life orb is more powerful.
(BTW gligurr, ik your answer is good. It's just you didn't calculate the life orb calculations properly and you had no hard evidence)