one of the most difficult things to do with a large collection of music is to navigate around it. Neither of the solutions you've described will help you here. As there's no screen at all with the caddy, navigation will be "blind" and the Elonex is described as woeful for navigation.
Although I'm a great fan of Creative and this: click here would fill the bill - perhaps with one or tow of these: click here to ease connection to your boombox
but you could get an iPod with one of many speaker systems with an iPod dock