If it is the 899 vs 999 series, the 800 does not have the 800khz frequency that the 999 does. I think the transducers are different and of course the screen size. If you are talking 898 vs 998 it is just a screen size difference. The I pilot unit operates independently of your sonar, but the Ipilot link can be connected to any ethernet capable humminbird unit. Just in case I wasn't clear, there are 2 minnkota gps control units. One is an Ipilot and the other more advanced unit is an Ipilot Link.