have used each before, about a mth each.
on the same pair of test earphones (sony G73s)
ipod vs iriver ihp120, double batt life, louder output, plays more formats, more EQ settings, better chinese support. but more expensive.ipod vs muvo2 is small but still hdd-based ie no excessive shaking, else longer batt life, cheaper, louder output.
ipod looks cooler, great if u on mac, elegant interface, an can engrave ur name on the back.
louder == bigger headphones usable, and more bass if u like.
batt longer == obvious + can stay louder longer.
capacity 20GB is too much for mp3. hell to manage.
size, if u considering ipod then u wun really bother about size. but ipod takes a WHOLE pocket w/o the slip-in case.