If a Saudi-Iran conflict were to occur in a vacuum, the war would not be about territory or regime change by force. Neither side can take the fight across the Persian Gulf, much less seize and hold strategic areas in adversarial territory. The conflict would be about inflicting damage to both punish the other side and compel it to cease hostile behavior. While the Saudis — with their superior air power, access to foreign military technology, and far greater wealth — might be better situated to endure such a conflict, if not impose greater costs on the Iranians, the Islamic Republic has less to lose and has shown an ability to withstand years of warfare against greater powers.
However, it is unlikely that such a conflict would involve only those two parties and not grow to involve other states. Iran lacks state allies (except for Syria, of course, which is hardly a state now), but it does have a robust, transnational alliance with nonstate clients in Lebanon, Syria, Iraq, and Yemen. Groups such as Lebanese Hezbollah or Asaib Ahl al-Haq in Iraq would almost certainly support Iran in such a conflict, including by targeting Saudi nationals in their own countries, but they couldn’t attack Saudi territory militarily with any degree of effectiveness.
Saudi Arabia, however, has a strong alliance with Arab states (especially the United Arab Emirates and Jordan) and with the United States. Were such a conflict to occur, it is difficult to imagine that the United States would not become involved in one way or another in support of the Saudis. Although Iran could certainly raise the costs of American involvement by targeting U.S. naval vessels in the Persian Gulf directly or by targeting U.S. forces and nationals in other countries by proxy, Iran would have to balance such actions with the risk of drawing the United States into a more extensive war.