I have kept both... regular nassarius are smaller and grey shells. Do a bang up job on keeping anything that hits the sand clean. Biggest I have seen is about 1/4". Tongans that I have also kept are white and about 1/2 or so large, also do a good job. As far as which is more effective or $ efficient, I think you get more bang for your buck with the tonga variety as they are larger and will eat more so you can keep fewer. Still in a DSB I think the smaller and more numerous would be better.