Why would Apple TV at 48 kHZ be inferior to Airport Express at what seems to be a slightly lower rate?
Because there is some loss of quality on 44.1kHz to 48kHz conversion.
Karavite, If data is identical then the only thing that change sound is timing. If you send music in form of S/Pdif stream from computer to DAC any time variation (time jitter) converts to noise. In case of wireless you send data without timing, most likely in packets. This data is saved in the receiver buffer (in my AE couple of seconds) and new timing is created either for internal D/A converter (if you use analog outs) or external DAC (if you use digital out). Since your computer timing is unimportant you can use slow computer, any HD drive etc. The only way computer can affect the sound, in this configuration, is thru the ambient electrical or mechanical noise in the room. In that respect faster computer can be much worse (more electrical noise, stronger fans).