I hate to sound like a smart alec but the one that will sound better if all things are being equal can only be determined by YOUR ears.
But if I had to choose which of the above options to have, it would be a dedicated pre/pro instead of a receiver ACTING as a pre/pro. One reason is that dedicated pre/pro doesn't have to share internal power suppplies with alot of other circuitry that isn't going to be used. In addition a dedicated pre/pro may have higher output voltages at the outputs than a receiver might. This means a stronger signal for your external amp to work with.
Which setup are you considering going? What components?
