funny, I've got both.
They sound really good, but the samsung galaxy s wins by quite a margin ESPECIALLY when used with larger harder to drive headphones. With iem's you can get away with using the n8, but with stuff like the ed8, denon d2k, akg k550- the voodoo sound on the samsung really smashes the n8.
For me, the samsung replaces a desktop rig to a large degree- that's how good it is!
My n8 had usb otg that worked with the fiio e7/e9, that allowed it to use the fiio's dac and amp to output sound. Note that I say 'had' and 'was'- the usb otg is severly buggy and mine virtually died completely.
Oh. and the n8's mp3 software sucks. I've tried them all and they either crash regularly or are too low on features to be useful. Very unlike the sgs