On the paper, at least Iran seems as a country more worthy of a partnership. Iran is a more moderate and modern country with more educated population. Their diplomats rarely get caught abusing staff in the western world or smuggling drugs. Proxy groups they endorse are arguably far less extreme and target mainly Israel and Saudi Arabia. Saudi Arabia on the contrary is a leader when it comes to terrorism exports to the USA, both in the ideology and the actual funding, private or state sanctioned.
Currently geopolitically Saudi Arabia is much more important to the US than Iran is. American companies have built oil refineries, rigs and pipelines to help oil extraction in newly created country. Now USA has multiple bases in Saudi Arabia to project their power in the region as well as to protect the oil extraction and exports in the event of regime change or other instability. Majority of their advanced military hardware is operated solely by Americans. To add the Trump angle to it, wealth disparity in Saudi Arabia is much higher and the country is much richer. As such Saudis are far more likely to buy flats in his fancy towers than any Iranian investors.
Saudi Arabia is a dictatorship that gives what US wants, Iran is less of a dictatorship that might not always give what US wants. Shift from supporting Saudi Arabia to Iran would be step into unknown with a payoff potentially much lower. It is impossible to predict if in the far distant future US could build multiple bases in Iran, expand their presence to central Asia through them or be a major supplier of weapons sold to Iran.
Much of the Iran’s treatment is due to an outdated rhetoric which will take a while to change. Media portrayal as a rogue terrorist state likely to start WWIII, stems from their actions against Israel and an open defiance against US dominated world order. While Saudis publicly condemn Israel they never act against it. Although Iran pursues/pursued their own nuclear programme we know that US can be happy with a far less stable country such as Pakistan having a nuclear arsenal. Iran is also on good terms with Russia. Russia supports Iran in the UN and the UNSC, they sell them goods otherwise under sanctions while Iran sells them oil at a below market rates. This again shouldn’t be much of a problem as US does not mind Pakistan being cosy with China.
That being said Saudi Arabia is a white elephant in the room for the western world’s long term interests and their special status should be slowly diminished. Iran has the potential to match Saudi Arabia’s oil exports as well as having much larger market to buy goods from the US for their industry which is much more diversified and technologically advanced. Best long term strategy for US would be playing Iran of against Saudi Arabia as well as having another big oil exporter around so that the chances of another unified oil embargo are much lower. I believe Obama has begun that process and it will hopefully continue following next administration change.