Hi Gus. “Better” is kind of a relative term I guess. I guess that you’re asking which will yield better results – true? My answer would be that as far as results go, it probably doesn’t matter much. If I assume that the workload is essentially the same with both methods, then the results should be essentially the same.
Where the “better” might come into play is in areas such as which would hold my/your/the client’s interest more, and thereby lead to greater exercise compliance. In that regard, my opinion would be circuit training. I love interval and circuit training for its variety and intensity.
I hope that this helps.
A set routine would be easier to track progress so it could be more motivating and easier to keep good form. On the other hand, circuit training would make your heart work faster. For overall calorie burn and hormonal response circuit training would be more intense because of the number of different exercises and the shorter rests.