I resisted getting into the DI/SI game as long as I could. I couldn't fish the way I do without 2d color sonar and GPS mapping. It took some arm twisting from friends and family here to take that step.
I was a Lowrance guy when I tournament bass fished in a previous life. I've been Humminbird since I got into the more advanced stuff. I started out with a 788 then added an 898. When a good deal on some used units came along, I went to 2 898s then to a 998.
I've bought a 1199 and a Helix 10 for the new rig and will have a learning curve to be able to use them as comfortably as my previous units.
My advice would be the same as others, buy as big a screen as you can afford but consider what you want to do and see with the unit. I have zero desire to have SI on the bow of my boat so I saved several $$ by going to the Helix 10 with GPS/DI. The 1199 is DI/SI so I had that covered already.