I use MetaGeek products for troubleshooting RF interference, as well as looking deep inside wifi data. I just upgraded to their Enterprise suite, and wow, its amazing. Tonic is really powerful, and you can really get down and dirty with it. You can also look into RF interference with their other tools such as chanelyser (you will need their Wi-Sly adapter for that though). I have two USB wifi adapters, and that allows the software to scan 5ghz on one of them, and 2.4ghz on the other.
and no, im not their sales person... LOL.... but their tools really are that good. Even export to Wireshark using their own Metageek filter for Wireshark.
I think anyone that has to manage a large wifi network really should have the tools to troubleshoot.
Here is a link, they do have demos of the software. Oh, just know that InSSIDer will be going away - Tonic is replacing it.
@Saravana_inflow is testing two APs it in same environment. Of course it could be that Sophos AP is using different/clearer channel than AP305C, but I would't be so sure that it could bring such big difference. Therefore, before you buy spectrum analysers, just go and check WLAN config, as I already suggested 🙂
You're right about one thing. Everyone who commercially does WIFI deployments should have spectrum analyser. That's undeniable.