[matt1]

If you ask for the credit card number up front, you'll increase the number of people that stay users once the 15 day trial is up, but you'll drastically reduce the number of people signing up for your service.

For a new app, I recommend that you let people sign up without entering their credit card number. You need lots of feedback right now and you won't get it if you ask for the credit card number up front. Plus, getting people to fork over their credit card number will force you to spent a lot of time on your site's design so that it appears credible, which is not something you should be focusing on yet.

If your app takes off you can consider taking the credit card number up front, but until then, get it after the 15 days is up.