I doubt there is any difference.
For Mac/iPhone/iPad it is well documented that sound is transmitted as AAC 256 kbps (and I don't hear the slightest difference between them).
For the Apple Watch I have not found hard data but I'd be surprised if Apple suddenly deviates from its audio standard only for this single device. [/assumption]
The optional headphone cable converts analog signals to 24 Bit/48 kHz and feeds that digital signal into the APM so theoretically there should be an improvement versus Bluetooth if the source material is high enough.
Of course if you store/stream music in a lower bitrate on/to the Apple Watch to save space/traffic the resulting quality on the Airpods decreases accordingly.