Just personally I tend to regard retort as short and reactive while rebuttal as a longer and more considered disagreement. A retort could be defensive and wrong or it could be sharp and insightful - it doesn't imply one or the other. A rebuttal is mostly an attempt to correct something while a retort doesn't need to be a correction (although it could).

Even something like "piss off!" could be a retort, but usually never a rebuttal :)