I've stayed at the Imperial and Bristol and walked into PH on its opening day. I've also walked past (and decided not to enter or ever stay there) the Sacher many times. The Sacher seems overrun with tourists (think cheap bus tour groups) and does not have the atmosphere of a top hotel. In fact, I'd be surprised if it were a genuine five star property, although I don't believe the Bristol is five star either. My advice would be to avoid the Sacher; definitely do not pay a premium for it.