What is the difference in the 500,600, and 700 humminbird sonars as the screens seem to be the same size and they all have some that are side image down image and have the 2 d and color. I am hoping everyone gets the newer sonars and I can buy a used unit after Christmas(real cheap) when they sell their older units. But I don't know what or how to know how to know the better deals to get. The real big screen sonars will probably be still out of my price range. They are getting to be so many of these sonars and a lot are also discontinued. Does anyone know a rule of thumb for buying a used unit or how to know the newer dated and better units? There needs to be a Kelley Blue Book for fishing sonars.:banghead
