The following concern 2012 versions of both:
Litespeed fits much trimmer. I wouldn't put more than a trim 200 weight fleece under the Litespeed, while a light down jacket fits under the Houdini no problem. Torso length is comparable, Houdini sleeve length is a bit longer. The Houdini has the cinch cord in the back of the hood, which makes it more adaptable than any other windshirt discussed here.
Both have excellent DWR. Breathability and wind resistance is pretty comparable.
FWIW, the Rab Cirrus has a slightly more generous fit than the Litespeed, longer sleeves than the Houdini, and excellent DWR. I find it less breathable than either the Houdini or Litespeed.
(Yep, we're doing a windshirt SOTMR.)