In my opinion, I think that the DHF is more useful. The reason for this is because this document is able to show a detailed and comprehensive design and development process until its release. Every change, every choice taken towards the design of the device, every mistake and every path taken is in the DHF and anyone that open the DHF is able to see how the device evolved from beginning to end. Every reference needed to make the device is in there, compared to the tech file in which it is just a snapshot of the design. It does not really tell you what changes where made and why, plus it does not contain any required SOP to follow. The DHF also, demonstrates that the design was developed in a coherent quality plan and a coherent set of requirements.