Like everything in life, it's all relative.
I started crappie fishing about 7-8 years ago with Humminbird's Piranha 170. Basic 2D sonar that helped me find my brush piles once I got close to the vicinity. At the time I was content, as it was all I knew.
Later found I could take the guess work out of finding the brush piles with Humminbird's 597 with GPS and DI. That was a HUGE upgrade for me. Ability to save waypoints and the image like pics of DI over 2D was a game changer. Little later I realized the potential of Side Imaging and thought SI was a must have and purchased the 698 HD SI DI. I found the 5" screen was a real disappointment when viewing side images. Basically found the screen too small to really see much detail. Not impressed at all. Me and my buddies fish one man kick boats and they have the 9" & 10" screens. You would think we were on two different lakes. No comparison. Last year I bought the 10" Helix and once again was really impressed. The SI reveals so much more on the larger screen that I just couldn't see with the 5" on the 698. My experience was money not well spent. Hope your experience is different. I fish 3-4 times/week exclusively for crappie so I'm counting on SI to find structure. IMO the 5" screen was fine for everything except SI.
But as I said, it's all relative, cause now I think a 12" Helix G2N would make a great birthday present for the wife (who doesn't fish).