Some misinformation appears above...
the USB connect has the potential for better performance since the Airport will convert everything to Apple Loseless
Apple Lossless is exactly that - lossless. There would be ZERO performance gain or loss. Both 802.11g and USB 1.1 have enough bandwidth to stream cd-quality audio so this is not an option. If anything, the Airport Express has an 8-second buffer that USB does not so there's additional protection against network dropouts if you're streaming audio in from the net.
Indeed the USB route is limited to 16 bit (redbook CD).
Neither USB nor 802.11g is limited to 16-bit/44.1 KHz. Both can handle increased bitrates, should that ever become important. It has absolutely no relevance in today's world, as the high definition formats are completely encrypted all the way to the DAC. That means there'll be no digital streaming of SACD or DVD-Audio anywhere!
some of the best DAC's are designed (intentionally) for 16 bit
Actually, all of the home/hi-fi DAC's are designed primarily for 16-bit input (even the dCS). It's only in the pro world where you would need to handle 24-bit streams.
Coaxial methods being preferred by many as the better way to stream digital info
With jitter correction circuits you won't notice the difference. I've tested USB into an M-Audio Audiophile through a $100+ Chord Prodac vs. the Airport Express and the Monster Cable optical and can't tell the difference.
I'd suggest a USB DAC like the Benchmark
The Benchmark DAC-1 doesn't have a USB input, and when I asked them about this earlier in the year they had no plans of adding one. I'd bet their space in that case is too cramped to add any more boards.
The bottom line is that the quality of sound you get out is in direct relation to the quality of the DAC. Everything else in the middle is pretty much irrelevant except when it comes to convenience.