Is there a reason why I would want to instantiate a form class rather than an SCX or the other way around? Is there more overhead involved using DO FORM rather than NEWOBJECT() ?

I do not know if one way has more overhead than another.

I think it is pretty much a matter of personal preference - except, of course, if you are using member classes and trying to access custom properties in the form's init(). there is a bug in the scx that does not allow the form to see these custom properties of member classes during the init(), but it works fine in a vcx based form.