Is it better to Tiamat before or after using an auto attack reset, with FoB?

Using Tiamat right after an auto and before an auto attack reset is incredibly fast, wasting almost no time with the Tiamat animation. However, using it after an auto attack reset means you have 2 more Fervor stacks, which means more AD, thus more Tiamat damage. It also means Tiamat will come up slightly slower later on. It also takes more time.

So? Which is better?