well if you want a portable that will be used almost entirely for music i would strongly recommend an ipod touch (not an iphone because of cost, service contracts, etc.)
if you want an all around portable that can do theoretically do almost anything you want it to do, then get the pandora
really the only things i see better about the iphone are
1. Form factor but this hardly counts seeing as the pandora has the best form factor for what it does but still the iphone is much easier to carry around (going by comparing my ipod touch in my pocket rather than my ds fat and ds lite)
2. Amazing development software
This is my personal opinion but really using Xcode, Interface Builder, and Instruments really makes it amazing to develop for the iphone
it literally took me an hour to have a fully functional latin translator for school that can do anything i want it to do
but you have to jailbreak and hack the iphone/ipod touch in order to get those apps that you develop on it, unless you want to pay apple
Also, i have not tried developing anything for the pandora (seeing as i only know the Cocoa framework and nothing else, come on i'm young i haven't had time
) so this really isn't a fair argument and is very biased
Oh and the pandora absolutely kills the iphone in terms of battery life seeing as i only get an hour of playing Metroid with minimum backlight, wifi off, and fully charged
what it comes down to is that both these devices really can do the same things but the iphone has better commercial apps while the pandora has a lot more freedom and horsepower