Invoicing App weigh-in.

Some of you probably know this but I'm developing an online invoicing app. I'm looking for a little feedback on which feature you would prefer.

Duplicate Invoice: Duplicate any invoice for a customer, tweak as needed.
-----or------
Invoice Templates: Create an unassigned invoice, tweak and assign it to customers as needed.

Which feature would you prefer, and, would you even use the other one?

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