Depending on the time of day or simply for peace of mind I´d cut the road (bus) sections off my itinerary as much as possible.
As long you are not too familiar with public transportation in Berlin I suggest taking the bus no. X9
or 109 to Jakob-Kaiser-Platz then go downstairs and hop on the U7
(underground) to Rudow (final destination) go back to street level and catch bus no. X7 or 171 straight to the terminal at SXF
This would cover 90% of the distance by underground on a single line with only TXL
to Jakob-Kaiser-Platz and Rudow to SXF
on a bus both taking less than 10 mins each and running frequently.
The underground part takes 45 mins and BVG calculates between 1h05 and 1h10 for the whole journey including walking distance/waiting at both bus/underground interchanges.
So, if you calculate 1h30 for the whole trip you´ll be on the safe side!
/109 as well as X7/107 run in 20mins intervals each but are scheduled so to offer a 10min interval between airports and underground interchanges (example: that means if you just miss the X7 at TXL
the next bus will be the 107)
Ticketwise there´s no difference between any (reasonable) route you may choose.
covers all (City)-Zones ( A+B+C / so, fare will be 3,00EUR for a single ticket)
It´s a possibility as this one has one less change but involves a walking distance from Schönefeld rail station to the SXF terminal of approximately 7-8 mins - not very convenient if you need to "schlepp" your luggage from a long haul flight. Even though there´s a shuttle bus from the rail station to the terminal I don´t see any real (travel time) advantage to that routing ...[Edited 2012-02-26 03:36:27]
[Edited 2012-02-26 03:37:09]