I've not heard the new iPod nano, but I'd take the current shuffle over the fuze for most earphones out there: it has the same great resolution with dynamic earphones, but has lower hiss and better stereo separation. If we got into the whole mah per output talk, then the only things that would sound good are DJ systems and speaker amps. Power only really serves to push earphones to louder volumes. That is why big home amps can push high Ω phones to loud volumes - they push more into the speakers. But at the same volume, the same amount of power has to be going into the same speakers. Naturally, higher volume can push up S/N ratios, but it if the nano or shuffle is loud enough already, there is no reason to push up the volume/power. What will make a player sound thicker or sweeter is its SoC and/or its output caps.
So, if the new nano sounds anything like the shuffle, then it sounds good. I'd take it over the Fuze. The fuze is a great player if you hate Apple. It is a great player for Rockbox. But it is not as clear sounding as a lot of players out there and somehow dissipates that amazing battery rather fast.