One thing I'll note about the Raymarine black-box packages is that they actually have 3 different sounder boxes out now. One that does up to 600 watts (DSM30), one that does up to 1000 watts (DSM300) and their new DSM400 which does up to 3KW! But for the sake of this conversation we'll say you are using the DSM300. That unit and the Furuno 585 have digital filtering technology, which is what helps picture definition and to get deeper. The 585, however doesn't have quite the screen resolution (in terms of pixel count) as the E series. This means you'll get a nicer looking display of your fish-finder on the E-series-- basically you will have a more pixelated, or blocky display with the FCV585.
In regards to the advantages of the FCV585, you are getting a standalone sounder, which means independent controls. This means you'll have less screens to flip to to get to your sounder because it'll always be on display-- with the E-series you'll have many soft-key operations that require a few more button pushes to get to the same screen. You also have some knobs to more readily adjust things like your gain.
The differences are fairly small when you consider the actual performance of the two, but remember that if comparing the two you should put apples to apples so this comparison assumes you are using the same transducer for either machine. If you have the B260 (a bigger, non-flush 1KW 'ducer) on the Furuno, but the B164 (a smaller, flush style 1KW 'ducer) on the Raymarine then you are probably going to notice a performance difference between the two quite quickly, but side by side with the same transducer, you probably will not notice a great difference in performance.
Let me know if you have any further questions and perhaps describe your application a bit more and we can go into further detail (if this isn't enough detail for you already
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