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Some rooms at the Bristol are nicer than some rooms at the Imperial, but the opulent suites at the Imperial are in a class by themselves (however these would not be available for upgrade and are expensive to match their lavishness). The Bristol would generally have nicer rooms and greater chance for an upgrade. At one time the Bristol was considered to be a better hotel than the Imperial (before it's refurbishment), and it has an arguably better restaurant (as well as historic ties to the musical establishment in Vienna). Have no fear, you are still staying at one of the top-20 city hotels in the world.