My ancestor passed away on Purnima. Do I perform Shradh on this day or on Sarvapitri Amavasya?

Ideally, you should perform Shradh on both days. The Purnima Shradh is the primary day for those whose ancestors departed on a Purnima Tithi. However, Sarvapitri Amavasya (the last day of Pitrupaksha) also covers all tithis including Purnima, so it serves as a valid backup if circumstances prevent observing Purnima Shradh.