Efficiency isn't the only factor. I'd say the most important factor is enjoyment. We each have a finite amount of willpower. If you're using it up on exercise y

Efficiency isn't the only factor. I'd say the most important factor is enjoyment. We each have a finite amount of willpower. If you're using it up on exercise you hate doing eventually you're going to hit a day where you don't have much willpower, skip the exercise and break the routine. The most time efficient exercises I know aren't ones you can sustain for longer periods of time. I can consistently walk for hours but only run sprint intervals for a few minutes. Sure every minute of sprinting burns more than a minute of walking but if I can walk significantly longer than I can sprint it will probably lead to a higher total calorie burn.