The first is the same as the second if you insert the word "the" between "representative" and "class." Thus: "The representative of the class." However, "class" is one off those nasty English words that have multiple meanings, such as social class (workers, nobility, bourgeoisie), a roomful of students with a teacher (science class, math class). There is at least one meaning I can think of where your first usage would be correct and the second wrong: if by "class" you mean elegance, quality, or style in a person. So, "Cary Grant was the representative of class" would be correct, and the second would not.

I think both

The 2nd one.

the second one, however mind that the first statement is actually grammatically correct, however the second form is more commonly used.