Simply put for me.... I always try to use the less toxic of two ingredients if there is a choice. I always try to use the most manageable product if there is a choice. And so, Photo Flo 200 is least toxic and most dilute. I can use 1 drop as an example (1/25th ml with my calibrated dropper). With Photo Flo 600, which is toxic, I would have to use 1/3 of 1 drop. That would be impossible requiring lab dilution of a toxic substance.
There is less handling of a virtually non-toxic substance as opposed to handling of a toxic substance in concentrated form.
I hope this puts it into perspective.