I would like to agree with all who have recommended the Sachertorte at Demel over Sacher. (This can be a contentious subject.)
The thing is, no Sachertorte is really all that good, from the cake perspective (and I would emphasize this even more if your frame of reference is an American layer cake.) There is a reason Viennese (and Hungarian!) cakes are served with
Schlag, i.e. whipped cream. As a very, very wise friend once quipped, Sachertortes taste like day-old Ding Dongs. To get that reference, you need a familiarity with the lower end of prepackaged American baked goods, but aside from the rather substantial difference in chocolate quality, it's a pretty spot on description. The thing the Demel Sachertorte has over the Sacher Sachertorte is that the Demel version is a bit moister.
I lived in Vienna, and one of my indulgences when I worked nearby was grabbing the bite-sized (probably two or three bits, really, just a small square) Sachertorte at Demel to accompany a light lunch at
Trzesniewski ("unprounceably/unspeakably good"). I could get the tiniest 'splash' (0.1L?) of wine with some bread and pate. It always made for a lovely lunch, and is quite Viennese.
Also, Sachertortes make for good presents, as they come beautifully packaged in a wood box. But you don't need to necessarily spring for the Sacher/Demel version for the same effect (particularly if you are giving these to co-workers or family who know nothing of Vienna), as there are plenty of places that sell perfectly respectable (read, still dry) cakes packaged in the box. And since the cake is already pretty dry to begin with, buying a few, putting them in your checked luggage, and distributing them some time later does not markedly effect the quality.