The assumption that a democracy is less aggressive and better governed than a dictatorship is quite widespread, and if wrong may lead to wrong interpretations of international security situations.
Thus my 50 cents...
There's always a difference in regard to civil liberties (although not necessarily for all; minorities may find their right to vote pointless, and a semi-democracy may even withhold suffrage from a minority such as convicted felons etc.), and this is clearly in favour of a democracy. This is no indicator for a government's aggressiveness, though..
The mechanisms of loyalty is a better indicator for this than civil liberties in my opinion.
A classic monarchy (think: European middle age, Pharaoh's, Chinese emperor) rests its legitimation on a divine order illusion. This has clearly lost its effectiveness during the last thousand years, and enlightenment did a lot to finish off the legitimacy of most such monarchies. Ideologies such as Fascism and Bolshevism developed into substitutes for the 'divine' aspect, but they proved to be much less durable.
A classic monarchy had a huge advantage over more modern dictatorships with its imaginary divine order; it did not require much effort on part of the dictator. The loyalty was mostly for free (the reformation changed this for a while; monarchs and princes had to fight for their divine monopoly during the 16th and 17th century).
Modern autocrats rule differently, mostly through two immensely wasteful approaches:
(a) Terror. Stalin and later Mao developed this path. The terror wasn't only a direct terror, but largely a scapegoating and ever new waves of purges, crisis, scapegoats - permanent revolution, with permanent turmoil. Extremely wasteful.
(b) Cronyism. Autocrats have the wealth of their country under their control, and they buy the loyalty of special interests in order to maintain a critical mass of supporters. The others suffer accordingly at the hands of such a redistribution of income and power.
The latter method (government buys special interest group's support with wealth and positions of power) extends into some politically unstable democracies as well.
Nevertheless, a democracy CAN (but does not necessarily) rest the loyalty of the people on the democratic legitimation of the government. This CAN (but does not necessarily) free the society of wasteful political efforts aimed at loyalty-sustainment.
A democracy (or rather: Republic) can swing to the other extreme as well: Special interests can begin to buy the loyalty of the government - a reversal of the modern autocracy pattern. This, too, can be extremely wasteful.
Wars of choice fit into the category "wasteful government activities" and can easily be launched by a monarch (most unpredictable), an autocratic ideologue (somewhat predictable thanks to the published ideology), modern autocrats (if it enhances, not risks, the loyalty of the targeted special interest groups), stupid modern democracies and corrupt republics (especially if war serves powerful special interests).
The true, non-corrupted and non-cronyism-riddled democracy can be expected to have the least systematic inclination towards wars of choice. It may still be stupid enough, though.
It's too bad that the usual pundit comments on whether a democracy or a dictatorship is more dangerous in international affairs do not look at the issue at a greater resolution. I think it's necessary to take the loyalty mechanisms into account or else the patterns won't be significant.
edit: Could have been written much better. It happens.