Neither are polite, really. "I don't care" is a statement of fact. "I couldn't care less" is even more emphatic: it is not possible, under any condition, for you to care less than you do now. This is the absolute bottom of your caring.

Definitely agree with Peachey. Both are impolite, however, the latter is a bit sassy in tone and often is said to make a point (i.e. "I couldn't care less if Mary came to my wedding or not.")