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).