i have the 6000ub
i was like you and didnt know if i wanted the 6000 or 600
there is no differance in specs at all they are both 4v @ 100ohm or something close to that
anyways they are the exact same just the 2 year warranty vs the 1 year
if your like me you buy a new head unit every 3-4 months so warranty doesnt matter
the 6000 and the 600 both charge the ipod and have really great control
you will like the list button where you can find songs by the same artist by holding down the list button
the usb is great and very fast
if there is anything else you wanna know just ask