In theory, both do the same thing of reducing No3 and po4. Question is which is more effective. End of the day it boils down to how big either one u are running. Its not fair to assume a big refugium is more effective than a small ATS. If given the space, i will run a refugium as it has grows more beneficial organism that will benefit the overall health of the tank. If space is an issue, than running an ATS will help for sure.