The most important metric is revenue. Can we get anyone to pay for the app. We have 250+ people sign up so far, but didn't have to pay us anything... Can we convert 25 of them to pay us $6.99/mo? Or will it be 2 of them?

Update... so far, no sales. But we have a queue of trial recommendations to fulfill, which we expect is a prerequisite to people paying us.

If we can't get people to pay up front, we'll switch to allowing them to upgrade, but not billing them until X days after signing up.

Whatever it takes to ethically maximize conversion-to-paid ratio.