My vote is for the Oriental also - my opinion is that the Peninsula is on the wrong side of the river - traffic on the bridge is often at a standstill although the Pen does have a boat shuttle across the river.

Must see sights for one day - definitely the Grand Palace (suggest hiring a guide from the hotel). When I'm in Bangkok, I never try to do more than one thing a day since it usually becomes impossible with the traffic and the heat/humidity will get to you. Also, you will be in rainy season which means rain just about every afternoon/evening.