True circuit training (no rest between exercises) is a form of cardio. IMO, if you had to choose between "traditional" cardio (e.g., running, eliptical, bike, etc.) and circuit training you should choose circuit training because it will not only get your heart rate up but, to some degree, also build lean muscle mass. In real life, you should have a program that incorporates some form of resistance training (not necessarily circuits) with some form of cardio and a healthy diet. It's up to you to experiment and figure out what works best for you.
'07 e90 335i: Monaco/Lemon ZSP ZPP ZCW (gone)
'02 e46 325ci: BSM/Black ZSP ZPP 5MT