As ktlemba said, the classes are not very effective if you don't spend some serious self-study and problem solving in parallel.
If you insist to take a class, take Manhattan over Veritas for MATH section.
Manhattan books are way better organized and explain with more details, while Veritas books are just collection of practice problems (exception: Verita's Combinatorics and Probability book is the best on the subject). Many people are attracted to Veritas shinier "reputation" but as a tutor I get a lot of students that want to improve their scores after a Veritas class and I can't remember a student that came to me after a Manhattan class.
Keep in mind that during the classes all prep companies just read their textbooks, you will get very little private tutor time to analize your performance and guide you.
My students tell me Veritas books are better in Verbal but I don't tutor that and can't comment further.
The best approach would be to get the
MGMAT guides for math, self-study, problem solve UNDER TIMED CONDITIONS, and consult periodically with a good tutor that can give you long homework sets for practice - 100 problems per topic is optimal to hit all necessary angles of a topic.