The damage boosts offered by Life Orb and Expert Belt are identical for SE attacks.
Life Orb is around to offer the bonus damage to swtiches.
edit: Danyul, with few EVs in speed, it would seem Life Orb Tyrannitar is being outsped by just about everything in the first place. Losing a few points of speed in exchange for keeping your physical/special defense is a solid play.
Life Orb
Holder's damage is boosted by 30%. The holder takes damage equal to 10% of its max HP each turn it uses an attack move. Rock Head does not block the damage caused to the holder. Life Orb does not damage the holder if the attack hits a Substitute. Life Orb damages the holder even if it uses Dragon Rage, Night Shade, and Seismic Toss, but does not boost their damage.
Expert Belt
When the holder uses an attack that is super effective on the target, the damage is increased by 20%.
If you have 29 HP IVs on the other hand, and you use Life Orb you can deal 11 hits instead of 10 because the Life Orb recoil is rounded down.