I don't think it is especially important to do a workout in a circuit format vs a cardio followed by weights format. A benefit of the former might be that you would keep your heart rate in a fat burning mode for a longer period of time.
But one could also create counter arguements to say that in the latter mode you might burn more calories overall. For example, in circuit mode you might work the cardio a bit less hard following a leg weight segment due to fatigue. Or, when doing all weights, you might lift heavier and work harder because you are not fatigued by the cardio.
I think with regards to this specific training question, you should do which ever you prefer.
