Like you said, one is granular and one is liquid. That is the difference. I wouldn't try to read anything more into it than that. As I understand it, the liquid is for the rafts of the broken free fil.algae that is floating, and the granular is for prevention of same from happening.
I have used the liquid in the past, but I don't do it now. Personally, I'd rather put in $45 worth of Aquashade every month to keep out the demons, and so far it has worked. I do keep a pointed stake handy on the dock, in case one of those demons decides to rise up out of the depths and surface, thereby scaring the women and children, but generally I intend to stick with the aquashade as a preventive.
Good luck with it.
