One way in which both of these excellent novels can be considered to be similar is the way in which they both, to varying degrees, attack the central notion of imperialism that lies on the belief that one group of humans is actually superior to another. Of course, European Imperialism is based on the premise that white people are superior to other types of people, who are "barbaric," "ignorant" and "savage." Both Conrad and Salih implicity question such easy binary distinctions in their text. Note how Salih does this in the following quote:
I preferred not to say the rest that had come to my mind: that just like us [the Europeans] are born and die, and in the journey from the cradle to the grave they dream dreams some of which come true and some of which are frustrated; that they fear the unknown, search for love and seek contentment in wife and child; that some are strong and some are weak; that some have been given more than they deserve by life, while others have been deprived by it, but that the differences are narrowing and most of the weak are no longer weak.
In spite of the differences created by culture, money, geography and history, this quote points towards the essential unity of all mankind, and shows that whatever distinctions are created by humans themselves to justify their actions, actually, at a core level, humans are all the same. Note how these sentiments are echoed by Marlow:
The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses than ourselves, is not a pretty thing when you look into it too much.
The "conquest of the earth," in Imperial mythology is defined by other characters in Conrad's novella as bringing the torch of civlisation to the darkness and ignorance of the rest of the world. For Marlow to describe it in such honest and brutal terms undercuts such rhetoric, and explores the way in which colonialism in reality is about the exploitation of those who are only slightly different from ourselves. This is one central comparison you can make between these two texts with regards to imperialism.