I agree, TXL is nicer and easier to acess. However, FRA has a direct raillink into Erfurt. Don't know whether you are aware the AB has flights very reasonable priced from DUS to Erfurt departing from DUS at 8:20 and 19:20 and Erfurt at 10:25 and 20:35. Considerung the shorter flight time into DUS, the easy airport and the 60 minutes flying time between DUS and ERF this could be an idea.