We compare the thermal properties of THz quantum cascade lasers (QCLs) fabricated with metal-metal optical waveguides based on Au / Au or In / Au wafer bonding. This information was obtained from the analysis of microprobe band-to-band photoluminescence spectra measured on devices operating in continuous wave (cw). The experimental normalized thermal resistances (R_L<sup arrange="stagger">*) , show that the use of Au / Au wafer bonding optimizes the heat dissipation. Comparison with surface-plasmon based THz QCLs, demonstrates that the use of metal-metal wafer bonding can allow cw operation at progressively higher temperatures.