In active/standby, one device handles traffic while the other waits. Simple failover, no coordination needed, but you pay for idle hardware.
In active/active, both devices handle traffic simultaneously. Better resource use, but you need load distribution and must handle failover of ongoing sessions.
Choose active/standby when: simplicity matters, stateful inspection makes session migration hard, or traffic volume is low.
Choose active/active when: you need the capacity, protocols support it, and you can handle the complexity.