Re: Choice of processor

    According to Microtype latest manual, LuaTeX for PDF production lacks:
     Kerning
     Spacing

As does XeTeX, and those features "not enabled" in pdfTeX. Nothing has
"Expansion/manual" enabled either. Anyway, none of that is important in
practice.

The crucial thing in that table is that "Expansion/automatic" is enabled
(green star) for PDF output of current luatex, while it is
"not available" for XeTeX. Which is what I stated before.

It's just one entry in that big table (attached for the heck of it, from
microtype.pdf for version 3.2a, 2025/02/11), but it's the only
difference that matters in practice for normal documents (e.g.,
everything I've ever typeset for TUGboat). In my experience.

If you actually run some sample documents with \usepackage{microtype}
under both xelatex and lualatex, you will see the difference. --best, karl.