K. A. Milton, Julian Schwinger (1918-1994) (PDF)
A paragraph with some relevance to the point you were making:
"Schwinger learned from his competitors, particularly Feynman and Dyson. Just as Feynman had borrowed the idea from Schwinger that henceforward would go by the name of Feynman parameters, Schwinger recognized that the systematic approach of Dyson-Feynman was superior in higher orders. So by 1949 he replaced the Tomonaga-Schwinger approach by a much more powerful engine, the quantum action principle. This was a logical outgrowth of the formulation of Dirac [21], as was Feynman’s path integrals; the latter was an integral approach, Schwinger’s a differential. The formal solution of Schwinger’s differential equations was Feynman’s functional integral; yet while the latter was ill-defined, the former could be given a precise meaning, and for example, required the introduction of fermionic variables, which initially gave Feynman some difficulty. It may be fair to say, at the beginning of the new millennium, that while the path integral formulation of quantum field theory receives all the press, the most precise exegesis of field theory is provided by the functional differential equations of Schwinger resulting from his action principle."