See Alan's answer.
By the way: there is no direct relation between total message count an number of received airframes.
Sometimes I've a high message count but a lower total number of flight movements and number of receivedairframes.
Sometimes it's opposit. And sometimes it matches up: higher message count equates higher flight movements and number of received airframes.
Further due to weather and other reasons of the chaotic system "global flight traffic" it's not enough to let the box run for a few hours to determine if one antenna is better then another. If you want do real check you need two (better three) boxes, a month of time and some calculatin skills. Futher you need both antennas set up at the same time.
How to do it:
Week one: Box A on Antenna A, Box B on Antenna A
Week two: Box A on Antenna B, Box B on Antenna A
Week three: Box A on Antenna B, Box B on Antenna B
Week four: Box A on Antenna A, Box B on Antenna B
Write down number of received airframes (count if date is selected im MyFlights browser) and number of flight movements (count in ANRB report) for each box and day of the week.
After you have that date it's time for some statistic calculation that I'll only provide if there is real interest.
Please refer to the attached image to see why a simple change antenna and watch message count won't really work.
Ingo