Wide band gap (WBG) devices and particularly Gallium Nitride (GaN) switches are attractive solutions to increase the compactness of power converters in automotive applications, thanks to their improved switching characteristics and lower on-resistance. Nevertheless, their 650 V voltage rating limits their use in 800 V on-board chargers (OBC). In this paper, a side-by-side comparison between two isolated 800 V dc-dc topologies is proposed, backed by consistent experimental data. These topologies are based on 650 V GaN-on-Si devices and derived from the dual active bridge (DAB): the series-input series-output dual active half bridge (SISO DAHB) topology and the three-level active neutral point clamped DAHB (3L-ANPC DAHB) topology. This work compares the trade-offs between these two power circuits and evaluates the impact of the switching frequency and transformer design on the GaN switches losses, the overall conversion efficiency, as well as thermal dissipation. Experiments conducted on the realized prototypes show very good efficiencies even at high switching frequencies. Slightly lower losses are observed in SISO DAHB compared to 3L-ANPC DAHB, that has a greater switch count but fewer passive components.