This really does depend on what your goals are and ideally an even balance of the two will get you good all round fitness, increased strength and endurance, improved stamina, good core strength and improved flexibility.
Cardiovascular fitness classes aim to raise the heart rate and keep it there for the duration of the workout, such as running, circuit training, cycling and swimming. If you were to solely concentrate on Cardio work you would see many health benefits, as it strengthens the heart, improving its pumping efficiency and reduces your resting heart rate. It strengthens your lungs and improves your lung capacity, boosts your metabolism, helps to lose weight by burning fat, and reduces stress levels.
There are many different ways of Weight Training and again, this comes down to what you want to get out of it. To gain muscle size you need to perform an exercise class with heavy weights, only aiming to get between 5 - 7 repetitions out, as well as taking on board a lot of protein and carbohydrates, to help repair the muscles and let them grow. The pace of these workouts is usually slow as you need your recovery time between each set. If you’re looking to tone up muscle then you would lower the weight and aim to get between 8 - 12 repetitions out, and by reducing your recovery time between sets you are working the cardio side as well, as your heart works harder to pump the blood round.
It is important to work all muscle groups, and not just concentrate on the vanity muscles i.e. arms, chest and shoulders. Dead-lifts, Squats and Bench Press are the important three. During fitness classes these exercises use large muscle groups, and release muscle growth hormones, so any other muscle groups you work, will also benefit from these three.
Circuit training is ideal because as well as giving you a cardio workout you are also working all your muscle groups with resistance training, as you perform exercises such as press ups, squats, dips, lunges etc and the repetitions can be high, as it is body weight resistance. By changing the body part you are working, the muscle groups just worked have time to recover as you move onto another body part. With minimal recovery time the heart is kept at a very high rate throughout the exercise class.
Steve