I like the 400: an adventurous appearance and strength of the device (without any irony), the simplicity of its menu, the accuracy of GPS. I do not like the lack of visibility of the screen in the sun ...
I love the 600: the speed of satellite acquisition always with that GPS accuracy, brightness and visibility of the screen, more powerful touch capabilities such as zoom by spreading the fingers,
I fear its less protected part of the screen (no raised edges like on the 400) a slightly more complex menu
In short, the 600 is of course more modern and efficient, the big improvement it was his screen, the 400 hardiest disappointing by his screen will still be less afraid of my bike ...