Audit trail is for transaction type records and captures 3 actions : Create, Change and Delete. Where system notes captures every record type (just a few exceptions) and capture every single changes to the record including context. Suppose you need to find an invoice's or customer record's full life cycle system notes will tell you every time it was modified/changed including GL impact. Where as audit trail does not have that much details. While back Netsuite compliance principal Keith Goldschmidt told me that for compliance reason they capture every details in the system notes. Note that searching system notes is a monstrous task, you have to have defined criteria and timeline otherwise it will timeout. Tools like FastPath and Flodocs uses system notes to generate complete audit trail, SOD and change logs.