Difference between XeTeX and LuaTeX

Problem

I have a hard time contrasting XeTeX and LuaTeX. At the moment, I am using XeTeX almost exclusively, mainly because it uses UTF-8 as the native input encoding and because it supports TTF and OTF fonts. However, the lack of support for pdftex’ microtyping is mildly annoying. LuaTeX, on the other han…