The plane count, message rate, max distance are dependent on:
(1) The Air traffic.
The air trsffic is not constant and varies with day of week and time of the day. You must measure these parameters for both antennas at the same time.
(2) Location & placement of two antennas.
If you want real comparison between two antennas, mount two antennas at same location, at same height with horizontal distance between the two not less than 30cm and not more than 100cm. Then use 2 dongles plugged into 2 rpi and compare results.
Above is called side-by-side test.
Please see photo below in which I have conducted side-by-side test for comparing V-Stub antenna with Flightaware Antenna.
I have used two (2) RPi and two (2) dongles, one set of Pi+dongle for Flightaware antenna, and the other Pi+dongle for V-Stub antenna.