Lowrance Elite 4 DSI ... :

DI (455kHz/800kHz)
Chartplotting
320 x 240 3.5" color display

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Humminbird 581i HD DI ... :

2d (455kHz/200kHz)
DI (455kHz/800kHz)
Chartplotting
320 x 640 5" monochrome display

Some people find DI as a "replacement" for 2d...Lowrance choice
Some people find DI as an "compliment" to 2d...Humminbird choice

Rickie