A different world. Mykonos = iconic Cycladic glamour island with beach clubs. Halkidiki = authentic Greek peninsula, family-friendly.
Mykonos: cocktail €15-25, sunbed €40-150/day. Halkidiki 3-5× cheaper. See budget.
Mykonos: Paradise, Super Paradise (party scene). Halkidiki: Sithonia's turquoise coves with pine forest backdrop.
Halkidiki: 1 hour from SKG. Mykonos: JMK flight or 3h ferry. Halkidiki 50% faster.
Mykonos: world-class. Halkidiki: lively but family-friendly.
Halkidiki clearly wins. See family guide.
Glamour + unlimited budget → Mykonos. Everyone else → Halkidiki.