Hotel Tango
I don't think the two routes are really comparable, as TXL doesn't even manage daily service, so won't be attracting higher yield business traffic in any numbers, whereas DUS is a multi daily operation, and apart from the much greater industry / commerce base in the Ruhr, the airport has good connections to other German and European business centres.
How much of a mess the move from LH to 4U actually makes of the route remains to be seen, but I'm sure Flybe will soon clean up on any point to point business that 4U manage to mislay.