I just got an Elite 5x-HDI (without GPS) and best I can tell is the price of these units dropped like a rock when the new "Chirp" Sonars came out. The 5 or 7 refers to screen size in inches measured across the screen like you'd do your TV. I got mine for $180 along with free shipping online at Cabela's back before Christmas. Go for it and we can compare notes. So far I'm quite impressed with how easy it was to set up and use. Going from an old Eagle Ultra II to this I've jumped well into the future with the down scan images! At default settings I'm seeing balls of shad, brush piles and clear structure off of a transducer mounted on the trolling motor foot. Oh, and one more plus - it comes WITH a transducer. The new ones come without and the transducer is another 100 bucks on top of the $1K unit. Pffft If I ever drop $1K on a fish/depth finder it better come with a transducer.
Some buddies have the Garmin fish/depth finder/plotter and they like theirs too.
Have a Great Day,
Jerry A