I believe comparing the two documents and ranking them in terms of more usefulness might not be the best because the DHF is used widely by the FDA and Technical File and Medical Device (MDD) are for the Medical Device Directive. Both documents are methods of design control. The biggest difference is the FDA looks are the history of the device whereas the MDD looks the safety and efficiency of the current design. Depending on the companies goals, the route can be through either the FDA or MDD, but each document is tailored to the methods of each agency.