When you say plays just mp3's do you mean you can't have a player that does anything else like voice recording or radio? I love sansa's especially the fuze because 1. Expandable micro sd slot and 2. Nice button layout 3. Rocboxable! Rockbox makes a much better firmware than the original sansa (http://www.rockbox.org/) I would go for the fuze, and don't forget because they are expandable you really don't have to worry about the player it's self having that much space.
What is a good mp3 player that just plays mp3s, with an exceptional battery-life and has a screen so you can see what songs you select?