Hoagie11

I need some advice on fishfinders and chartplotters. I want to upgrade my cheap electronics on both my dash and at the bow. But I need to stay within $600 for both together. So here is my line of thinking.... I'm leaning towards the the 3.5" display DI "only" at the dash to locate structure, brush piles, bait, fish, etc. idling with the big motor. Then get the 3.5" DI with GPS for the bow so I can fish with the DI and mark waypoints as I fine tune the location, brush pile, etc. with the trolling motor. Am I thinking correctly on this??? Is there any reason to have standard 2D sonar if the DI is so clear and gives you a better picture? Do you prefer Lowrance or Humminbird? Any other advice will be greatly appreciated. Thanks in advance!