Neither. The function of the Mechanism is clearly to represent the positions of the “celestial bodies” relative to the “celestial sphere”. The reference observer is situated on the surface of the Earth. For this purpose, the Mechanism necessarily has to use a geocentric model.
This does not, however, mean that its maker necessarily favoured a geocentric model of the universe. Aristarchus of Samos (c. 310–c. 230 BC) was the first known Greek astronomer to propose a heliocentric model of the Solar System. Supporters of such a model encountered considerable opposition, both in antiquity and in later centuries.
It is quite possible that the designer of the Mechanism was aware of the possibility of a heliocentric universe, but the instrument itself does not demonstrate this.