Dictatorial and Imperial are two government types in Stellaris. They each provide bonuses for an empire and each can give access to Civics that are not available to the other government type..
The main difference between the two is that Dictatorial rulers are individuals who have forced their way to the top, while in an Imperial style of government leadership is inherited from the ruler when he or she dies. Both governments have rulers for life. Rulers in these government types have absolute control over everything.
In both government types Rulers come with their own agendas that can’t be changed, plus the Ruler keeps them for a lifetime. One example of an agenda could be Xeno Outreach, which increases the Xenophile Ethic Attraction by 10%. Another one would be Scientific Leap, which gives a 10% bonus to research speed. Only the Rulers in Dictatorial, Imperial, Oligarchic or Corporate government have agendas.
Both government types provide access to the Philosopher King Civic. This civic makes it less likely for your rulers to acquire negative traits plus it provides a +2 in the Ruler Capacity level.
When the leader dies in a dictatorial government the game chooses four potential leaders for you and if there’s one you like you can pay 250 Influence to make him or her your leader. If you want to save your Influence the game will randomly select one for you. This style of government reduces the effects of empire size by 10%.
A Civic you gain access to with this government type is the Shadow Council. This reduces election costs by 75%, the Ruler Pop resource output is increased by 10% and codebreaking skills are increased by 1.
In an Imperial government you always have two rulers with only one occupying a leader’s slot. When the current leader dies, the second leader becomes the ruler with another ruler being randomly chosen to become the next ruler. The leader who is next in line also gains experience points, something you don’t get in a Dictatorial style of government. This style of government gives an extra point to power projection.
An Imperial form of government gives access to the Autocratic Elite Civic. This Civic replaces some Politician jobs in the Capital Buildings with Nobles. It allows you to create more Noble jobs by constructing Noble Estates. It also allows access to Noble Chateaus, which increases the Governor Level Capacity by 1.
Another Civic an Imperial government makes possible is Imperial Cult. In this society there is a dominant state religion, and the Ruler is worshiped as a supreme being. It gives a bonus of 100 to the Edicts fund.