I always buy a Flex ticket when arriving at FRA. I have been as much as 12 hours late and 1.5 hours early (super tailwind). The flight that was really early I had a Sparpreis ticket for a flight 90 minutes after landing, so I was looking at hanging around for 3 hours. About an hour after landing, my scheduled train posted a delay so I was able to change to the next train without charge.
If I have checked luggage I take the bus to T1 as it leaves right outside baggage claim. If I have only carry on I usually take the Skyline over to T1 -- as others have said it is more walking but generally quicker. Since the Skyline is both landslide and airside you can clear passport control in T1 (or at least you used to be able to)