I can heartily recommend the Hotel Bristol. I had a wonderful stay there a couple of months ago. It isn't far from the Hotel Imperial. Although I haven't stayed at the Hotel Imperial, I visited the lobby there, and it appears impressive.
Back to the Hotel Bristol...As an SPG Plat (for which the OP qualifies), the Hotel Bristol takes good care of guests. The rooms are nice. The location is great. The decor in common areas, such as the breakfast/dining area, is really nice, in an art deco way. I can't say if the Hotel Bristol is better than some of the other luxury hotels available (Sacher, Hotel Imperial, Park Hyatt, etc.), however I liked it enough that it would be my first choice if staying in Vienna again. The Hotel Bristol is one of my favorite hotels in SPG's portfolio, yet might still come in 2nd place to Vienna's Imperial, in some people's estimation. The room pricing is also somewhat reasonable compared to other good hotels in Vienna.