I've got both (mmmm shineeeeee) and the 20Gb one's the one to have - you never have to worry about shortage of space on it - - unless you're bloody Jay of course, in which case you get a 1Tb model
Who'd a thunk it, warning somebody off a 4Gb music player cos it's too small! It is very nice though.
Practically, stuff like the iRivers and the new ones from Creative are better in terms of functionality & battery life etc, but -let's face it- the Pod's the daddy
Get yerself one of those iTrip gubbins and you can tune your radio into the Pod too - abroad, naturellement.
Sorry, can't help on the JBL side of things - funky though eh?
Downside of the Pod by the way - mine's a year old and I'm having to replace the sodding battery already. Still, they're down to about £30ish nowadays.
EDIT: As per Gav's comments - apparently there are new earphones out for the iPod, supposed to be much better. That said, they'll still be crap and a walking advertisment for 'I've got an iPod, please pull out a knife and nick it off me', which is why I've got a nice set of BeyerDynamics which sound absolutely brilliant.