I have two 798's and have to weigh in on this. Having come from a Lowrance 527 that had better base mapping I can say I still miss the extra detail it had. I use the SI for snooping around in areas I don't know, which is most everywhere I fish these days. The DI is a great tool that really shines in deeper water, say over 10 or 15 feet, for keeping track of what the fish are doing. The sonar does a good job but I have gotten used to deep jigging trees and watching the white flecks give me a better look than the sonar offers IMHO. I would get the biggest screen I could find a way to afford. One day soon I will buy a 998 and sell the 798 just for the bigger screen. Easier on these old eyes.I have a riser on my forward FF and that helps a lot but nothing beats a bigger picture. Good luck with the decision. There's so much good equipment to choose from.
Creativity is just intelligence fooling around