Personally I think this is classic chevy vs ford debate.
I have a HDS 8 with the structure scan add on and I am extremely happy with it. It def took some getting use to but you CAN most definitely use the Lowrance version to find bait fish and fish.
I may be wrong on this but I was under the assumption that Humminbird uses 'software" to get there downscan image... whereas the transducer the airmar made for lowrance actually has 3 separate elements; left right & down. The free upgrade from HB is just a piece of software that helps create the downscan image, The lowrance Structure Scan kit (LSS-1) comes ready out of the box for left, right and down. Also the way I understand it, if you want to add a second head unit down the road for bow of boat etc you have to have a connection cable with Humminbird which is another 200 + or - a few bucks. The Lowrance can distribute to 3 head units out of the box. here is an example from another forum that did a comparison between the Humminbird downimaging and Lowrance Structurescan.
HB Down imaging 800khz
Lowrance Downscan 800khz
HB Downimaging 455 khz
Lowrance Downscan 455khz