Not with a DAC (yet). The wired sound is woolly - actually sounds very good on classical and instrumental, but noticeably poor definition with vocals.
I'd be surprised if there were much of a difference between the optical outs between the two, but I suspect the analogue out is better on the squeezebox, if you're happy to stick with non-DRM content. Airport Express can be fed by Airfoil, which allows any audio from your PC to get to it (great for BBC radio feeds). But then there's no remote - am going to investigate whether Apple TV can feed an AE, which might offer me that (not going down the iPhone/iPod Touch route just yet). You pays yer money......
But instead it just kept on raining
My useful posts: