Using Flow data is the recommended option since it allows to provide real-time traffic information to IRP without infrastructure changes and at a low cost.
Using Mirrored traffic will allow IRP to detect in a faster way network issues if the min_delay algorithm is enabled. This algorithm basically checks for TCP packet retransmits. In case it detects frequent retransmits, the destination prefix will be injected immediately for probing, therefore allowing IRP to react faster to these issues. Setting up Mirrored traffic can be quite expensive (10G link and NIC) according to the amount of bandwidth utilisation.
For best results, both methods can be used simultaneously.