Neither really, a better item for Scizor would be Scizorite or a Choice Band, depending on what set you have in mind.
I would reccomend one of these two, but if it had to be between Life Orb and Choice Scarf, then it depends on what role you want it to fill. If your team lacks a speedy Pokémon and you want a revenge killer, go for CS, although it does fine enough with priority Bullet Punch anyway.
If you want more power, go for Life Orb.
So it really depends, but I'm leaning towards Life Orb, because the most used move will probably be Bullet Punch, which already has priority.