Wasabi ... this is all I know :
In general terms, 455 kHz will give you a longer useful range than 800 kHz. ... Also, in general terms, 800 kHz will provide a slightly higher resolution image than 455 kHz.
I used the 800kHz frequency on my Lowrance Elite 5 DSI, and still use it on my Hook 7 ... image quality was/is better, plus knowing that the 800kHz has a 45deg angle tells me that when I'm in 20fow the beam is about 16ft wide at the bottom.