Parliamentary or Presidential?
There are different systems of government in use around the world today — some authoritarian, some democratic, and some totalitarian. Putting aside the undemocratic regimes, there is a common pattern to be seen.
Most democracies choose one of two countries to model themselves after: the United States, or the United Kingdom. Either they adopt the presidential system of the US, or they follow the Westminster parliamentary system of the UK.
Do not be fooled by the fact that some countries with parliamentary democracy have presidents, such as India and Singapore. In all countries which follow the Westminster system, significant executive power is vested in the Prime Minister, while the head of state, be it the monarch or president, plays a minimal role in affairs of state.
It is actually very interesting to compare the two systems of democracy, because both have so very obvious strengths and weaknesses. The element of "checks and balances" plays a significant role in both systems, but the much-vaunted separation of powers truly exists in any meaningful sense only in the American presidential system.
Under the Westminster model, the legislative branch determines who occupies the executive branch; the executive branch then determines who occupies the judicial branch.
Of course, the reality is more complex and subtle than that — the head of state appoints the head of the executive branch based on who he or she believes will have the confidence of the legislative branch. The Prime Minister can be toppled at any time if the legislative branch passes a vote of no-confidence against him or her, or rejects the executive's proposed budget.
The executive is normally responsible for appointing members of the judiciary; there is little, if any, legislative oversight. Some people, such as the Lord Chancellor in the United Kingdom, can occupy all three branches of government at the same time by holding a cabinet post, sitting in the House of Lords as a legislator, and sitting in the House of Lords as a judge.
The legislative agenda is usually set by the executive; not much time is usually given to bills proposed by individual members of the legislature (though sometimes the executive covertly introduces bills in this way to avoid drawing attention to them). Since typically the executive is supported by a majority of the lower house of the legislature (most Westminster democracies are bicameral, although some such as Singapore only have one chamber), the legislative agenda is usually passed without a hitch.
Under the presidential system, all three branches of the government are theoretically equal. The president has the power to propose laws, but the legislature is completely free to reject them, since the president does not need their support to stay in power. In the US, Congress has even shut down the government bureaucracy by refusing to pass a budget — much to the chagrin of the powerless president at the time.
The president nominates appointments to the judiciary, but these must pass before the legislature first. The legislature often sets its own agenda, even though the president may propose something; "pork barrel politics" is common in the US often because individual legislators decide funding, and so all insert provisions for their own special interests.
Already the strengths and weaknesses of each system are obvious. Under the parliamentary system, if the executive is performing terribly, the legislature can easily boot it out, either causing a snap election forcing the public either to confirm their support for the executive by electing new MPs more amenable to the Prime Minister's policies, or necessitating the appointment of a new executive.
If the same thing occurred in a presidential country, though, the president would likely hold on to power. In such a system, the legislature is normally wary of impeaching the executive, because it requires proof of some actual crime — you cannot toss out the executive simply for executing insane policies.
But at the same time, dissent is much easier to quell in a parliamentary democracy — you may let the dissenters have a voice in parliament, but they can rarely do anything, especially if the whip is tightly enforced on most issues, as it is in some illiberal Westminster democracies. There's a fighting chance for dissenters in a liberal parliamentary democracy, but since the government sets the legislative agenda, it normally tries to push dissent aside by allocating less time to it.
Under the presidential system, which often comes with a bicameral legislature, there are in effect three crucial areas which must be controlled for effective laws to be passed: the executive branch, the lower house of the legislature, and the upper house. Since all three are elected separately at different times, in effect this means dissent has a real chance of actually making an impact.
But, as always, these things carry double-edged swords. The track record of presidential systems is generally one of one stunning or not-so-stunning (depending on your view) success, and a host of failures (see: the Philippines, most of Latin America, Liberia, etc.). In countries like the Philippines, the presidential system has been blamed for legislative gridlock.
Parliamentary democracy has a much better history because of the stability it brings. But as nations develop, their people yearn for more freedom to dissent and permit the dissemination of more views — that seems to be why the UK itself has been moving in the direction of a presidential system; much of what the conventional Westminster system looks like does not really apply to the UK anymore.
I once conjectured that the reason presidential systems had failed in other countries was because these other countries had not adopted the strong federalism of the US. But it seems to me that this does not really explain the situation, as the people of developing countries, whether parliamentary or presidential in nature, have squandered the democracy they were given.
The stability of the parliamentary system, despite its authoritarian tendencies, appears to be best-suited for developing countries. But countries change with the times; a more democratic and plural governance system along the lines of the presidential system has to be implemented as a country and its people grow. Trying to force either system down the throats of a people as a one-size-fits-all solution may do more harm than good.