FWIW, I have a 3G Nano and a 160gig Classic. The Classic is nicer at the lower end, fuller without being too thumpy, but the Nano is much better in the highs- cleaner and crisper with way more definition. So it's really a toss up as to which sounds "better." I usually end up listening to the Nano with headphones and bumping the up the low EQ a bit, and plugging the Classic into my stereo.
That said, neither one sounds as good as my final gen Mini (which is going to be reborn as a 32gig flashPod!

)