Originally Posted by
dj_jay_smith
+1 for the train, but I would suggest book Rail & Fly if possible.
I prefer to take the train quite often for domestic German travel. However, I think in this instance, a flight is more convenient. He's already at FRA airport and TXL is so close to Berlin's city center such that a flight will save some time. A train takes 4:15h at a minimum.
From OP's statement that he plans on flying business, I don't get the impression he's looking for budget travel. I think leaving three hours between flights is conservative enough. The OP could also be a bit more aggressive and leave less time. If he misses the flight, he should be prepared to pay up to 300 or 400 euro extra in the worst-case scenario (only fare buckets Y/J open).