I don't think you'd ever prefer their A-slots versions over their Seals (and sadly, Flashing Blade is only available as their former). A-slots have much better and more skills to mix in with, like Fury & L&D. You can always shift around Seals, unless you mean you want both on the same team.
Flashing Blade's probably even worse as an A-slot. It seems easier to reach over 50 Atk than it is 40+ without Seals. Hell, even Mia (in my case, my -Atk one) is better off with Heavy Blade Seals, since it allows for L&D.