I have both dongles. The FA dongle is way better than the airspy without an amp.
They are about the same when the airspy is used with an amp, however, in my situation(busy area) I need an Odroid XU4 as airspy_adsb requires a lot of CPU.
I suspect the gain is too high or a filter is not being used.
(This is clearly stated in flightaware.com/adsb/prostick/ )
I had to cut my fa dongle down to 38db with a cavity filter to get the best results.
Without the filter my aircraft numbers went from 200 down to 2.
This thread has a gain optimisation script that is really worth running on any RTL-SDR dongle
ads-b-flight-tracking-f21/gain-adjustment-t37172.html