"Indent with tabs, align with spaces" (e.g. [1]) would be my strong preference in a perfect world. In addition to its "ethereal" benefits, some languages which support HEREDOCs have nice "tab ignoring" versions (e.g. <<-).

That said, in the imperfect world we live in, I always use spaces. "Indent with tabs, align with spaces" is obviously not rocket science, but its just too opaque unless you have a strong code review process.

[1]: https://dmitryfrank.com/articles/indent_with_tabs_align_with...