Idealism and materialism do not form an actual dichotomy of methodological choices but instead describe two bad ways of approaching history, and it's especially pronounced in approaches to American history.

Idealists seem to think that history is just moral philosophy by another name. Nations can be understood as principles, and the deviations from or corrections toward those principles. Materialists reduce systems to grinding mechanisms absent any human agency.

If you have to choose between the two (please don't), the materialists are better. They will at least give you some basis to develop from. Knowing the details of the plantation system, the volume of trade in the North, etc, is actually valuable context.

It seems obvious to me that the creation of the federal government was an incredible achievement by a relatively small number of people, and that those people had ideas about how governments should operate and what people's rights were and such, worth taking seriously.

It seems equally obvious that the range of possibilities was constrained by the way the interests of influential persons were structured at that time, and the fact that such persons were influential/powerful enough that they could not be ignored.

Moreover, these things are not separate. Some principles really are just rationalizations of standing interests. Some push against them. Some are neutral and considered for other reasons. Few are related to one another that cleanly.

My favorite mixture of bad takes on this, which still managed to be an actually informative paper, was one that defined the Progressive movement simply as rent-seeking, but treated the original Constitution as if it was written by philosophers https://engagedscholarship.csuohio.edu/cgi/viewcontent.cgi?article=1539&context=clevstlrev

Mostly it benefitted from the fact that hardly anyone talks about the politics of how the 17th amendment came about, and historians of the Progressives have an ironic tendency to be just as "idealist" about them as others are towards the founders.