Definitely retake. I took the LSAT on a whim in college with very little prep (one practice test, not under test conditions). I was cocky because I love logic games. I didn't even bring a watch to the exam (I kinda wanna shake the 21 y/o version of myself). I scored a 151.
3 1/2 years after that, I decided to pursue law. So, I prepped for the LSAT, retook, and scored a 166. Unfortunately, that 151 is still on my transcript for this one last year. However, because the jump was so great and it has been so long since that awful 151, adcomms seem to be looking past it.
Point of story, retake after actually preparing. Write a nice addendum to your application on why your score jumped 20+ points (mine basically just states that the first was with very little prep), and raise your standards a little.
There are some exceptions (Yale, maybe NYU), but most schools will overlook an old, low score if you have a new, high score.
Granted, I'm a 0L, but I really get the impression that it's much easier to retake the LSAT than to transfer law schools.