Jack,
Don't know what was wrong with me as I did not give you a very good answer. While almost all of the 700 seies share the same basic 2D sonar capabilities, some will have additional sonar capabilities such as the Si sonar (on 798 only) and the QuadraBeam capabilities (on many of the models). Other athan that the differences are in the display. Generally the more vertical pixels in a sonar unit the better the image but also the higher the cost as well.