to let you guys know the result, I could not resist the 1380 $ for a helix 12 chirp, bought a 360 trolling with it for less than 2200 $ total. Just installing it now and curious for the 360 imaging.
For 360 I thought the bigger the screen the better when casting and looking from a few feet, and though I saw some mega images, the results when the bottom is muddy is quite poor I heard (absorbing the high frequency) and I can see fish also on 455 and 800.
thanks for your replies!