Both men were gigantic egomaniacs who passionately believed themselves to have been sent by Providence to fulfill important roles in world history. The more powerful they became, the more convinced they were of the almost divine importance of their respective missions. They were both, obviously, thoroughly evil individuals who had no regard for human life and no compunction in sacrificing the lives of millions to fulfill their own warped, megalomaniacal obsessions.

Mussolini was less ideologically driven than Hitler. What constituted fascism in Italy was never, as a consequence, established with any degree of certainty or conviction. Policies changed with breathtaking rapidity, particularly in relation to foreign affairs. Hitler, however, was much more of a true believer in the ideology of National Socialism. His grand conception of a Nazi racial empire in Europe was markedly different in character from Italian colonialism in Abyssinia, which was largely concerned with the recovery of an imagined imperial greatness.

Despite his background as a fascist, Mussolini was less radical than Hitler. Fascism as it developed under "Il Duce" was generally a reactionary ideology used by conservative forces to counter the growing threat of anarchism and communism, especially in Italy's rural areas. National Socialism, on the other hand, envisaged a complete transformation of society along racial lines. This reflected Hitler's monomaniacal obsession with race, an obsession not shared by Mussolini.

I suppose the consolidation of power and increased control would represent both leaders' guiding principles.  They passionately believed that their visions for their nations represented "the path" and was vitally important for their nations' futures.  They both believed in eliminating opposition, by any means necessary and were fervent in their idea that they should embody their nations' present and future.  The magnitude of self was a principle and idea to which both leaders clung to with the greatest of zeal and passion.  There was little in way of planning for after their absence for each leader believed in the "now" and the idea that they embodied that particular moment.

For Hitler the ultimate goal was to control the entire world.  I do not believe that Mussolini had any such grandiose plans.  However, he was motivated by a desire to restore a Roman Empire of sorts (except he looked more to Africa than to Europe for his empire).

For both men, the guiding principle was fascism.  Fascism believes that a singler person should be the undisputed leader of a society.  This society should be as homogeneous as possible in terms of ethnicity and in political thought (it should be completely dedicated to the leader).