Koh Rong is one island with four very different fronts: the ferry village, the long empty beach north of it, the resort strip on the far west coast, and the outer bays where you trade shops and wi-fi for silence.
Koh Touch — the ferry village
Every scheduled boat from Sihanoukville lands here, and the guesthouses, dive shop, mini-marts and bars are packed into a few hundred metres of sand. Cheapest beds on the island and the only stretch with real services; also the loudest.
Long Set (4K) Beach — the quiet side
Four kilometres of near-empty sand about fifteen minutes' walk over the headland from the quieter end of Koh Touch, with only a handful of resorts on it. The nearest shops are an hour away on foot, so arrive with cash, water and whatever you need for the evening.
Sok San — the west-coast resort strip
A fishing village and a long beach on the far side of the island, home to Sok San Beach Resort and The Royal Sands and their own boat transfers from Sihanoukville Port Gate 1, under an hour away. Comfortable and self-contained; you will not be wandering to another bar.
Practical: there is no ATM on Koh Rong
Bring US dollars and riel in cash for the whole stay — Sok San Beach Resort states it plainly on its own site — and note that the outer beaches have no shops or wi-fi either. Ferries are the only way off and they stop in bad weather, so leave a spare day before a flight.