The flakes have to be cooked before adding to the whipped cream, and they take longer cooking to completely dissolve than the powder. It’s very tricky to add the warm agar to the whipped cream, as it starts to solidify at about 90-95F, so you have to get it in before that, which is pretty warm to be adding to cream. With some creams, adding something that warm will start the cream curdling pretty quickly.
The powder you can just sprinkle in without cooking first, or for a stronger stabilization you can cook it first, it will then have the same issues as the warm flakes. If you sprinkle in the powder, you need to use twice as much as you would cooked, i.e., 1/2 tsp agar per cup of cream if you’re going to cook/dissolve it, and 1 tsp of the powder per cup of cream if you’re just going to sprinke it in. Sprinkling is much easier 
I have to admit I don’t like using the flakes very much, maybe it’s just the brand available locallly or something, but it seems like they never completely dissolve, even with extended cooking.
