The emergence of gallium nitride (GaN) as a popular material for power electronics applications due to its superior transport properties has seen the need for developing enhancement-mode GaN high electron mobility transistors (HEMTs). Several techniques have been used to achieve enhancement-mode, i.e. normally-off operation of AlGaN/GaN HEMT devices, but there are only a handful of studies on the transport properties of such devices. This paper uses an advanced framework for modeling the mobility of the 2D electron gas (2DEG) in GaN HEMT devices to assess the performance of different types of enhancement-mode HEMTs. Three types of enhancement-mode structures are compared: an AlGaN/GaN HEMT with a p-type GaN cap, a double heterostructure Al-GaN/GaN/AlGaN HEMT, and an AlGaN/GaN HEMT with a p-doped GaN buffer layer. The gate voltage dependence of the 2DEG mobility at different temperatures is analyzed for all three structures and the key scattering mechanisms are identified. It is concluded that at room temperature, when polar optical phonon (POP) scattering is dominant, the p-GaN cap structure exhibits the highest mobility due to weaker confinement of the 2DEG, while the other two structures show a ~15% lower mobility. At low temperatures and high gate voltages, this trend is reversed when interface roughness (IFR) scattering is the dominant mechanism, because of the different energy dependence of inter-subband IFR scattering rates in the three structures.