Ran an experiment last night, I spec'd the beautiful & deadly Darth Phallon through a lightning and madness build comparison, and here are my findings.
Lightning Build: http://www.torhead.com/skill-calc#201ZfsrrzoMutzZcr0M.2
Madness Build: http://www.torhead.com/skill-calc#201Zf0bMZfrbRsMkrfz.2
In a 1 minute fight the lightning build did 52,824 Damage to the target. In that same minute, the madness build did 60,112 Damage to the target. To rule out lucky critical hits I ran the test multiple times, and found that Madness will consistently do approximately 7000 points of damage more than lightning.
The one thing I will say about lightning, is that the rotation feels very slick, much like a car with a very smooth transmission, you don't feel the gears clunking and you up shift. Madness has more of a truck like feel, but once you get used to it, the build has UNLIMITED POWAAAAAAA!
Also, Madness has way more mobility than lighning does since the only spells with a cast bar are Crushing Darkness and lightning Strike, both of which you can negate that with the Wrath proc.