Both are good units, I have both. I have the 320C on the bow and Fishmark 480 on the console. The 480 has more power, larger screen and a little more detail but the color unit is much easier to see. Push come to shove, I would say the 320C is a little easier to use/see, I would go with it.
My next unit wll have both high pixel count and color. If you can afford it, I'd consider a unit like 500C or 640C.
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Joe