Well, you need to decide for what purpose or role you want to use this card?
1. Do you want simple T1 or E1 configuration using CAS? Then libpri is not needed.
OR
2. Do you want ISDN PRI (Q.931 protocol), which uses CCS? Then libpri is required.
For SS7, you either use libss7 with Asterisk for Digium cards OR use mod_freetdm with Freeswitch for Sangoma cards.This is a recommendation not a requirement as i understand Sangoma cards are also supported on Asterisk and Digium cards also work with Freeswitch. Sangoma and Digium both are good competitors of each other. So they keep on developing and releasing newer / better hardware and its support on various voip switches and so on. Choose according to your requirements.
Thank you.