You don’t choose whether the spell is going to be an Aura spell or not until the spell is already on the stack. Abilities that affect when you can cast a spell, such as flash, will apply to the spell after you’ve made this choice. For example, an effect that said you can cast creature spells as though they have flash won’t allow you to cast a creature card with bestow as an Aura spell anytime you could cast an instant, but one that said you can cast Aura spells as though they have flash will.