We compared the thermal performance of mid-IR GaInAs/AlInAs quantum cascade lasers QCLs having identical gain medium, but different heat sinking configurations. By using a two-dimensional anisotropic thermal model, we have calculated the temperature profiles and the heat flow patterns of ridge waveguide QCLs, either buried or planarized, and mounted substrate-side or epilayer-side down. Device planarization with Y2O3:Si3N4 dielectric layers gives an 7% reduction of the device thermal resistance with respect to InP buried heterostructures. If this planarization is combined with thick gold electroplating and epilayer-side mounting of the device, the thermal resistance is reduced by 35% and 50%, respectively, with respect to conventional ridge waveguide structures.