So, I've decided to record these videos because more and more people recommend, when solo DC, to use FF over BtS because "it's faster".
On the left 4x BtS, on the right 4x FF:
On the left DG BtS DG BtS, on the right DG FF DG FF:
On the left BtS DG BtS BtS, on the right FF DG FF FF:
Sure, there are human errors and terrible editing skills involved, but still... I don't see any reason to use a 15% buff over a 30% one. And furthermore, it's faster to cast BtS with two empowered stacks (20% buff) than FF with three empowered stacks (15% buff). And no, FF does not have a wider area. And no, BtS does not affect only five allies. And no, slowing an enemy is not better than reducing its damage by 40% (I've heard that too...).