I am looking at new units. I already have a 565 at my console, I am going to put that one in the bow. I am looking at two different models, a 597ci HD DI or a 788ci HD DI. Other than $150.00 what is the difference? I like the down imaging and card slot for a map program and also GPS. Am I on the right track for a good upgrade? Any help would be much appreciated.