Ok - I've read this several times very carefully. Thank you all for your help. I just have a couple detail questions.

Editing to add - the website is awesome. I've been playing around with the interactive map. It does look like the X9 option is shorter but I can't plot that in the Route planner. Maybe because it's not the best way even though it looks shorter??

Avoid the S-Bahn. I can relate to that - one of the Boston subway lines is to be avoided if at all possible!

Option #1 -
1. From airport, get ABC ticket from Bus Driver for TXL Bus that says S+U Alexanderplatz via Hauptbahnhof
2. Get off at Hauptbahnhof
3. Take Regional Train on Track 12
Either RB7 or RE14. Question - in FLYGVA's post, the RBs and REs were mixed up. Does it matter?
4. If RB7, take towards Wunsdorf-Waldstadt, Bahnhof and get off at Schonefeld.
5. Question - if RE14, towards where?

Option #2 -
1. At airport get ABC ticket from driver and take X9 bus to Zoologicagischer Garten
2. Go to regional train platform. Question - is the train platform in the same station as the bus will drop us off?
3. Take regional train to Schonefeld - 3 questions - which train? Which track? Towards which direction?

Of these 2 options, is there a consensus of which is fastest? I know everyone has their own opnions about the best way to get anywhere but I thought I'd ask anyway.

And finally - if we are late landing at Tegel - should we just take a taxi?

And I really appreciate this help. I will also be asking about getting from Dresden to Munich - should that be a separate post?