Using eth and thorn to show the voiced-unvoiced distinction is basically only a thing in Icelandic and the IPA (and even then it's not a consistent feature of Icelandic), and when they were used in English they seem to have been basically interchangeable

That said if someone wants to bring them back to English it seems to me like using them to distinguish the sounds is the most sensible approach, it's the one that makes spelling less ambiguous even if it doesn't have a historical foundation in English