Well...at the risk of sounding like a "fanboy"....
I have had a Nano. I currently own the Z5.
FAR better battery life.
Load and view Pictures (while listening to music.
Better scratch resistance.
Dedicated volume control.
Better volume and sound quality (possibly objective).
Better interface. The Z5 interface is FAR better than the clickwheel, despite some reviews to the contrary. Just as easy to use and more functional.
No iTunes. The Z5 works well w/ WMP10 but will also work standalone. You can use it w/o even installing the software that comes with it.
Looks cooler. Purely subjective, I know.
I am debating between these two different flash-based mp3 players. I'm confused on which to choose. I want something that I can control with one hand. I like the Samsung because of the larger screen, better battery life, and stratch resistance, but the nano offers a wide range of accessories and the click wheel. The size isn't a major issue. I have seen both in person and the YP-Z5 is plenty small for me, but I am totally confused on which to choose...