My memory of mistake is fuzzy but I don't think its right that mistake has to be about terms?
The most fundamental difference is one of degree. A misrep can be relatively light and still be actionable. Mistake (both mutual and unilateral - the two are very different, its important to distinguish and keep them separate) has to be fundamental to the nature of the contract. Of course there are other differences (misrepresentations have to be made by the other party, the other party doesn't need to be responsible for a mistaken assumption to found mistake) but I think degree is the most important one. Mistake is a much rarer remedy than misrepresentation and much harder to prove as a result of the difference in degree.