I got to say, I have enjoyed adding SI/DI to my boat. I have Lowrances HDS so I'm not familiar with the HB numbers. For me, I opted for a larger screen.
I figured if I was going to spend the money and make the investment I wanted to make sure I had no regrets because of screen size. I think down imaging is pretty cool to look at but it alone wasn't going to make me spend the money. I wanted to have the SI. Now I must say I don't use SI every time I go fishing but I do like having it when I want it.