Packages of commercial Gallium-Nitride power semiconductors present increasingly small dimensions to enable low parasitic inductance, increasing the heat flux density of the package and the challenges associated with their thermal management. This paper compares between Printed Circuit Boards and Direct Copper Bonded as substrates for a Gallium Nitride based half-bridge from a thermal and reliability point of view. The layout for both substrates was numerically optimized and a compact thermal model is proposed to compare both substrates. Measurement results confirm the modelling approach. Both substrates were also analysed regarding their maximum operating temperature and expected thermal cycling capability.