I have just been through the same head scratching process. I originally had the 551s down on the list but in the end went for the Garmin 556s - the reason being that it has a 640x480 screen res instead of 320x240 - so there is four times as much info on the screen. I did this without seeing them side by side - and I won't see the finished product until September/October so I can't say whether it was worth the extra or not but I figured a 320x240 would struggle to show chart and sounder in any detail and an image I saw of an Advansea unit with the higher resolution looked much much better.
I wanted an Icom 421 but that is now only available in white and I wanted black, so I ended up with the Icom 411 instead but it was 50/50 between that and the Standard Horizon 1500.
A Boat is a hole in the water, surrounded by fibreglass, into which you throw money...
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