Both are barrier islands, both have genuinely good beaches, and they suit different trips. Here's the actual difference.

Siesta KeyLido Key
SandFamous quartz sand, stays coolGood, but not the quartz claim to fame
CrowdsBusier, especially the main beachNoticeably calmer
ParkingFills by mid-morning in seasonEasier, metered lots
NightlifeSiesta Key Village — bars, live musicQuieter, geared to St. Armands Circle nearby
Distance from downtown~20 min drive~10 min drive
Best forClassic beach-vacation energy, groupsCalmer trip, easy downtown access

Pick Siesta Key if...

You want the beach itself to be the main character — the sand is the reason Siesta Key has the reputation it does, and Siesta Key Village gives you a walkable strip of bars and restaurants without needing a car at night.

Pick Lido Key if...

You want easier logistics — parking that isn't a competitive sport — and you'd rather split time between the beach and downtown Sarasota or St. Armands Circle. It's the better base if a museum day or dinner downtown is part of the plan.

Staying more than 3 days? You don't have to choose — Siesta Key and Lido are close enough to visit both, so pick your base by this comparison and treat the other as a day trip.