I am in the process of buying a fish finder. Can anyone tell me the difference between the 800 series hummingbird and the 900 series hummingbird. I know the 800 series has a 7" dia. screen and the 900 has an 8" dia. screen. The difference in price is about $500.00. I want a graph that is I-pilot compatiable. I just know that some of you experienced folks will have more info. for me to consider. "Thanks" in advance.