If you can swing it go for the 8, the soft keys make it much faster and easier to make adjustments, unless the 7 is a touch. Arguments can go both ways for the larger unit on the bow or console. If the LSS is on the stern I'd put the larger unit on the console. If you just purchased the HDS-5 and SS it may qualify for a $300 rebate.

http://www.lowrance.com/en-US/specialoffers/