Heat load variations in district heating systems lead to increased costs for heat generation and, in most cases, increased greenhouse gas emissions associated with the marginal use of fossil fuels. This work investigates the benefits of applying thermal energy storage in district heating systems to decrease heat load variations, comparing storage using a hot water tank and the thermal inertia of buildings (with similar storage capacity). A detailed techno-economic optimisation model is applied to the district heating system of Goteborg, Sweden. The results show that both the hot water tank and the thermal inertia of buildings benefit the operation of the district heating system and have similar dynamics of utilisation. However, compared to the thermal inertia of buildings, the hot water tank stores more than twice as much heat over the modelled year, owing to lower energy losses. For the same reason, only the hot water tank is used to store heat for periods longer than a few days. Furthermore, the hot water tank has its full capacity available for charging/discharging at all times, whereas the capacity of the thermal inertia of buildings depends on the heat transfer between the building core and its indoor air and internals. Finally, the total system yearly operating cost decreases by 1% when the thermal inertia of buildings and by 2% when the hot water tank is added to the district heating system, as compared to the scenario without any storage.