Difference between XeTeX and LuaTeX

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 hand, does support this, as well as UTF-8 input and (rudimentary?) fontspec support.

So my question boils down to: Is there any advantage in using XeTeX over using LuaTeX? Has anybody used both and can compare them? In particular, LuaTeX sounds very experimental and unstable – but is this really the case?

From February? Been a while since I looked for latex qns here on SU, obviously...

Speed (Luatex can be kind of slow), older code base, and ease of use (You don't need to do anything with Xetex to use system fonts). I don't know of any feature advantage Xetex has over Luatex.

Xetex microtypography: from the Texlive 2010 pretest page:

XeTeX now supports margin kerning along the same lines as pdfTeX

So Xetex is a moving target.