Main difference is the Helix series units are stand-alone only, and the 800 series can be linked to another unit to share data and accessories.
The Helix 7 SI GPS would be the equivalent of the 899ci HD SI Combo as far as sonar functions.
$300 MSRP price difference.
Edited by Wayne P. (07/11/15 03:03 PM)
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, 360, RC-1