Doing a weight that you can do sets that last longer than 90 Seconds will put you in an oxidative energy state which means you will burn more fat. This can also be accomplished by doing sets that last 60 seconds with rest of no more than 30 seconds between sets. Use a superset format (2 exercises alternated, e.g.bench/pulldowns)for even greater results.
I like circuits of 8 exercises each 30 seconds with as little rest between exercises as possible. Rest 2 minutes between circuits. Vary you circuit each time you train to keep the results in high gear. Mix in heavy workouts occasionally for added variety and strength building.Personal Trainer Studio City