Both words have more to do with “feelings” than “reason”. In both cases, the individual’s heart rules his head. When you respond to a situation “emotionally”, you do what your feelings tell you to do. You don’t really think things through. In most cases, it is a knee-jerk reaction. When someone advises you and you react emotionally, you can either shout at the individual or you can begin to cry.
*The chairman became emotional and accused the accountant of fraud.
A sentimental person is easily affected by emotions like sadness, love, etc. Unlike an emotional individual, a sentimental one does not express himself loudly or strongly. The feelings that he expresses are much more tender and somewhat exaggerated. Sentimental often carries with it the sense that the feelings expressed are not genuine. Unlike “emotional”, “sentimental” has a negative connotation.
*Latha is very sentimental. She cried when she heard that the school dog was missing.