Koh Chang

The 999 bus runs from Ekkamai to the Ao Thammachat ferry pier in about 6 hours for ฿329, and minivans with the ferry included cost ฿700–900 from Suvarnabhumi or Khao San. Bangkok Airways flies to Trat airport (~1 hr), a 20-minute transfer from the pier. The car ferry is ฿90 and takes about 30 minutes, and our Bangkok to Koh Chang guide has every option.

White Sand Beach has the most hotels, restaurants, and nightlife within walking distance. Klong Prao suits couples and families wanting a long, quiet beach with mid-range resorts. Lonely Beach is the budget and party base. Bang Bao and the far south work best for divers and island-hoppers: you're at the pier, but a songthaew ride from the main beaches.

May and October are shoulder months, wet by Koh Chang's standards (Trat averages 420 mm in May and 338 mm in October against 85 mm in November) but green, quiet, and heavily discounted. June to September is a genuine gamble, with 777–1,024 mm a month. The southwest monsoon brings rough seas, snorkeling trips stop running, and many west-coast bars and bungalow operations shut for the season. If your dates are fixed in those months, I'd go for the Gulf islands like Koh Samui, which get better weather.

Koh Samui has an airport, more upscale resorts, and far more restaurants and nightlife. Koh Chang trades that convenience for jungle, cheaper accommodation, and a slower pace, and it's a bus or minivan and a short ferry from Bangkok. Beach for beach they're comparable. The deciding factors are usually flight access and how developed you want the island to feel.