Which is all around better though a Lightning build with Lightning Strike or Madness with Spamable Force Lightning not which is better for Madness?
Are we just talking Lightning Strike vs Force Lightning or are we factoring in the rest of the tree? If just LS vs FL, let's consider the following (using 2x LS vs 1x FL, since FL takes twice as long to cast) -
1.) Both are more or less free when specced up, and they even contribute to actually regenerating more Force if you're just spamming them.
2.) FL does more damage.
3.) FL has a slow.
4.) FL is channeled, rather than activated, so you can cut it off to move. But, LS only takes half the time to cast, so you can cast one, move / do something else. FL is somewhat front-loaded in damage, though, so you can do half it's damage in the first second.
5.) LS can proc Forked Lightning, which I guess is nice? (My Sorc is 39, but I've never bothered going high enough up the Lightning tree to mess with this).
If the question was "Given a hybrid spec between Madness and Lightning, what should I spam?" I'd say I prefer Force Lightning, because the damage / cost and damage / time ratio favors it, along with having a slow. But, there are of course other considerations when determining spec (bubble stun!).