Formalism is the jurisprudence of rules. Functionalism is the jurisprudence of balancing tests. If forced to choose between formalism and functionalism, I would probably come down on the side of formalism. I would not do so, however, because there is some meta-rule that prescribes formalism. Rather, it would be because formalism, on balance, has better consequences than functionalism – in other words, because there are good functionalist reasons to be a formalist.
Where I part company with many constitutional formalists is not so much over the desirability of rules as opposed to ad hoc balancingbut rather over the generality and the source of the rules we apply. Rules come in different levels of generality, from abstract "principles" on the one hand to specific "commands" on the other. Although some formalists believe that the Constitution can be described as a collection of commands – an "instruction manual" – most of the key provisions of the Constitution are best regarded as principles. This description is consistent with both the breadth of the language used by the Constitution and the basic purpose of such a document – to create a permanent framework for government "adapted to the various crises of human affairs."
Thomas W. Merrill,
Toward a Principled Interpretation of the Commerce Clause,
Harv. J. L. & Pub. Pol'y
Available at: https://scholarship.law.columbia.edu/faculty_scholarship/831