I would be careful calling the Flight Recorder a "Circular Buffer", this sets unobtainable expectations. If, you have an issue midway through a longer test (20 minutes), Avalanche Capture will not be able to capture this middle portion, only the beginning and the very end of the test run. You will have to mirror the ports on your device and use an external capture tool such as Wireshark.
No circular buffer for the Client/Server Packet Trace - it'll record the first 50 megabytes (or whatever value you configured) of traffic then stop recording. However if you turn on the Flight Recorder that feature will use a circular buffer, that is, record the last 1000 packets.