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 creature card in whatever zone you're casting it from. For example, an effect that said you can cast creature spells as though they have flash will allow you to cast a creature card with bestow as an Aura spell anytime you could cast an instant.
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This makes me wonder still.
Gotta know the steps for casting spells.
- Put the spells on the stack <--- Flash applies here
- Choose mode, alternative costs, kicker, splice, etc. <--- Bestow applies here
- Choose targets
- Determine costs
- Get mana
- Pay costs