Was early 19th century U.S. expansion "manifest destiny" or aggressive imperialism?

This is going to depend a lot on your definition of "manifest destiny."  To many people, the idea of manifest destiny is similar to aggressive imperialism and this would not be an either/or sort of a question.

Manifest destiny was the idea that God had essentially given the US the right to expand its territory.  The US deserved this because it was a superior country.  If God has given a country the right to expand, then it is appropriate for that country to engage in aggressive imperialism.

If what you are asking is whether US expansion was motivated by good or by the desire for power, I would argue that there were elements of both but that the desire for power was paramount.  The US, for example, actedly more humanely towards its Filipino subjects than Europeans did towards "their" Asians.  The US even promised the Philippines independence after a few decades of American rule.  However, the main point of taking the Philippines was not to help Filipinos, it was to gain access to resources and to have a military outpost in Asia.  Therefore, US expansion was more about aggressive imperialism than about helping others.

How did "manifest destiny" contribute to 19th-century American imperialism?

The idea of manifest destiny helped give rise to later American imperialism because it provided a justification for imperialism.  It gave Americans a way to argue that their imperialism was actually a good thing for those who were being conquered.  It gave them a way to claim that they were not just acting selfishly.

The idea of manifest destiny was that God had destined the US to expand over a large area of land.  God had done this because the US was a superior country.  It had a superior form of government.  It had a superior religion (not just Christian, but Protestant in particular).  It had a superior culture.  Therefore, it deserved to expand.

The same ideas helped to justify later imperialism.  It was good for the US to take the Philippines, for example, because Americans were superior to Filipinos.  They were Protestant where the Filipinos were Catholic.  They had democracy where the Filipinos did not.  For these and other reasons, conquering the Philippines would actually be helping the Filipinos.

In this way, the idea of manifest destiny provided a justification for later acts of imperialism.