hello sfvoyage,
you can check all three transport options on this webpage www.oebb.at or - when in austria by calling 051717 (both are austrian state railways info services). on the phone first choose 'regional travel', then 'vienna, lower austria and burgenland'.
if you are interested in modern architecture, you can see/spot the four converted gasometer buildings as well as günther domenig's late nineties 't-mobile' headquarter from the bus:
http://en.wikipedia.org/wiki/Gasometer,_Vienna
http://en.wikipedia.org/wiki/T-Center
in town, i would recommend you to take a stroll from the bus through the old town, yes: rupprechtsplatz, then fischerstiege, wipplingerstrasse, maria am gestade, hoher markt (especially great if you have watched carol reed's 'third man'), then the area 'behind stephansplatz', i.e. singerstrasse, franziskanerplatz, grünangergasse, blutgasse.
if time allows, i would take the s-train early, BECAUSE it has seven stops, in order to get off at 'zentralfriedhof' - the station is right in front of 'entrance 11' which leads to a section of this expansive 'central graveyard', which has some fantastic family tombs from the 19th century. maybe have a stroll here (it's evry picturesque) and go straight back to the train. this would obviously require that you have one extra hour 'to spare' on the way back to airport.
http://upload.wikimedia.org/wikipedi...r%C3%A4ber.jpg
useful taxi numbers are 01-60160 and 01-40100 - they offer discounted fares to the airport, if you ask for them on the phone.
have a great and very scenic stopover!