To test receivers under field conditions is a real pain and is rarely done properly, even at Lectro (!) in the past. To do proper receiver tests, all receivers must be on the same frequency, picking up ONE, I repeat ONE (!!!), transmitter and must use the same receive antennas with 2 two way splitters giving each receiver under test exactly the same signal at the same time. This removes transmitter antenna differences, RF interference differences and receiver antenna differences. Comparisons are rarely done this way and therefore always inexact (wrong).
Here are some things that we found that loused up our two transmitter, two receiver comparison tests big time:
This says that comparing two brands of receivers, transmitters, etc., is a crap shoot at worst and difficult at best. Of course, after being beaten unconscious by the sales people multiple times, I've learned to keep my mouth shut when a customer tells me how much better our equipment works than brand X. If you still want to compare two different pieces of wireless gear, I recommend multiple walks, switching frequencies multiple times, changing positions of the transmitters and moving the receivers around so that they occupy each others spots on different walks. You must also set the two transmitters up for correct or full modulation and then set the two receivers to have the same audio output level. To get valid results without the splitter setup and single transmitter described above, requires lots of tests and much walking.