@Andreas_B95 

Hi,

To me it looks like the only differences are the outer design and the type of antennas used. 
The Archer TX20U Plus uses a casing carried over from a previous model (Archer T4U Plus) with extension cable and conventional dipole antennas, while the Archer TX20UH features a newer casing design which includes a separate cradle with extension cable and also has a different antenna design, likely a printed circuit board antenna. 

However, for even better performance in combination with your AX73 router you might want to consider waiting for one of the upcoming models that support 160 MHz channel width, like the Archer TX50UH or Archer TXE70UH.