Both look similar, but they're very different:
Color longevity: Platinum stays white naturally.
White gold needs periodic rhodium replating.
Durability: Platinum displaces rather than wears away. Scratches can be polished out without losing metal.
Weight: Platinum is heavier—some love this, others don't.
Price: Platinum costs 40-60% more than white gold for equivalent pieces.
Maintenance: Platinum develops a natural patina over time. Some like it; others prefer to have it polished.
For chains you'll wear daily for decades, platinum's durability often justifies the premium.