This is one of those questions where you are never going to get a definitive answer because those with Zens will extol its virues and those with iPods will do likewise.
Each machine has its pluses and minuses.
The Apple is more expensive - the Zen is heavier and not as pretty.
The opposion cry "the Apple has battery problems and they can not be replaced". The Apple fraternity say if you do have a problem the battery can be replaced and in any event the warranty is now two years.
IMHO you need to go and look at each - see what they do and which one grabs you.
The iRiver is getting good reviews at the moment and the mini iPod is due any time.