571 HD DI...
The 571 HD DI features a high resolution 16 Level Grayscale 640V x 320H 5" display with Down Imaging® and DualBeam PLUS™ Sonar with 4000 watts PTP power output.
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596c HD DI...
The 596c HD DI features a Best in Class brilliant color 640V x 640H 5" display with LED backlight. Down Imaging® and DualBeam PLUS™ Sonar with 4000 Watts PTP power output...
Looks like the biggest differences are...:
596c HD DI ...color, higher pixel density in same 5" screen, LED backlit...
571 HD DI .... 16 level gray scale (B&W) , FSTN LCD screen...
I would think the higher pixel density should give sharper images... Especially when the 2d and DI are viewed in split screen...
Rickie