I've actually compared W550i and N70 (through Ad-15 adapter, on Powermp3 app), on Sony Fontopia headphones (which were included with W550i)

Believe me, even if there was a difference, no normal ear could seperate the two.

N70 could eventually go upto higher volumes, even beating Winamp's max on my PC. (due to the equalizer in powermp3 app, it present a Db bar that could further upper the max volume), on top of that, N70 now can play WMA and OGG formats (with 3rd party apps) - Both non-playable on any W series phone, bar that W950i symbian smartphone.

However, that was with a W550i, I haven't got the oppurtunity to test it with the newer Walkman phones like W810, but I don't expect any difference at all. And with Kingston's latest release, a 2GB Rs-DV mmc, space won't be a problem either.