Biopellets can be tricky to dial in and you can accidentally strip the tank of too much nutrients. Like I did :D. That can not happen with an ATS. I have run pellets in the past and always had challenges with stability. That's why I switched to vinegar dosing, more precise. I haven't tried an ATS cause I now have a refugium that takes care of that.
Also BP will remove nitrate proportionately more than phosphate so you will still need to address phosphates. IMO, based on what ive read about ATS they do better at removing both.