Like everybody has said you want to have two units if you can afford it. If you can't, you do what have to do to fish and have fun. The way I look at it, I have my cheaper unit in my dash to give me depth and water temp and to look while I am idling sometimes. I have my better unit (HDS7 Gen 2) up front to use while trolling and searching for new areas. The HDS7 has DS and SS which are nice but not necessary. I have a HDS5 in my dash. Both have GPS which is also nice but not necessary. That is my thinking best in the front. Other fisherman on here might have their best unit in the dash for idling around and scoping out new places without putting down the trolling motor. Some have killer units in both places. Good luck.