Since the PNR is supposed to expire after a while, make sure you call AA or check online for the ticket number.

Your account is good for one year from the original ticket date.

The implication of having an account credit is that future travel can only be in your name (or your name must be among the travelers), whereas the paper voucher can be used for someone else.

On the other hand, having an account credit on-file allows you to ticket future flights instantly, instead of having to mail in the voucher or ticket at the airport.