Without the combo:
Bloody Butterfly:
Spell casting can not be interrupted.
Increase Casting Time by 30%.
Inflict 5% more damage with Fire Wall.
Phen:
Skill casting can not be interrupted.
Increase Casting Time by 25%.
Basically the BB card will increase your Cast time by an extra 5% then Phen. So unless you have the Combo, or want to do extra
Firewall damage, Phen is probably the best possible choice. Edit: just to clarify: Increase means it'll take longer to cast.