Here are several tables and graphs comparing the economic performance of dictatorships and democracies:

Years in power vs. regime type:

The distribution of economic growth rates by regime type:

Growth rates by regime type vs. years in power:

Corruption vs. years in power:

The economic performance of leaders lasting more than 5 years in power, 1950-2002:

The entire workbook, containing other views of the data, is here.