I have stayed at the hyatt but not the Conrad, so can't compare service/amenties, but I do know the Conrad's location. For a leisure stay, the Erawan's location is far better: adjacent to lots of shopping and restaurants, and connected directly to the Skytrain. Wireless Road (Conrad) is nowhere near the Skytrain, and, given Bangkok's traffic and weather, that can sometimes make it challenging to get where you want to go.