I thought the 3 were:
Main- full flow. fully submerged so no air enters, thus is practically silent.
Secondary- takes up that little bit of extra that may happen once in a while- since the flow is so low it sheets along the pipe, minimizing gurgling sounds.
Emergency- only is in use when both the others are maxed out.
To add to that..the second drain is technically called an open drain. However, in the event that the full siphon fails, the open drain kicks in as a full siphon when the water raises to the tube that dangles just above the overflow's water line. If this happens you know you have a clog. Worst cases scenario, then the emergency kicks in. At 1.5 there isn't much in a tank that won't pass thru. An advantage to using larger pipes. When reducing the pipe size, not only can noise increase, but it decreases the passage way in which larger debris/object and get stuck easier.
The Bean Animal is truly a set it and forget it system. If power fails, there is no tuning necessary to kick back in. Once the pump starts back up, typically 30-60 seconds later the Bean has finished purging and is back to being quite again.
The Bean does take up real estate that is for sure. But the extra fail safe can't be matched. Good thing about the Bean is that its versatile in configurations. You have the traditional bean (side penetrations, you have external overflow boxes with vertical penetrating, you have durso style bean, 2/3 bean in which some run just a vertical emergency tube without elbows, I have seen herbie/bean hybrids (two hole penetrations but 3 opens).
The ultimate decision comes down to what you have space for. I have even considered notching out or passing thru one of my walls and then re-penetrating it lower on the wall just to incorporate the Bean. This would be for a 180g that I really don't have space for, lol. If no 180 build I will shave the 4 extra depth inches and make a 150g that won't require modifications to my wall.