If you are looking at an HD unit especially if using side scan, go with an hd8. They are just so much more user friendly than the hd7. You will appreciate the 1.5" larger screen using 2 to 4 window displays. Everything just gets smaller to see, you will thank yourself in the long run. And also you will love the touch buttons at the bottom of the screen. Just setting the overlays up on the screen makes life so much easier with the buttons, without having to go in the menu eveytime and pushing alot of buttons. I know it cost more but you will have this for years to come.
Between Low or bird, find a dealer that has both, then you can decide which one is the best for you. Size does make a difference.