Get the biggest screen you can afford. I got the HDS 8 Gen2's this year and love them. I'll be selling one of them at some point this year to upgrade to the 10 if you are interested.
It's actually a 2" difference between the 7 and 8. For me it was worth it but it could be different for you. I really like the big screen for running new lakes. We went to Squam for the first time in May and were able to run the lake no problem. The lake is full of hazards and my partner and I were able to zoom in on the navionics map, avoid trouble and ultimately save time. The bigger screen is also nice for doing the split screen when you want to look at structure scan, down imaging and traditional sonar all at once. Tough to see all of that detail with the smaller units. Is it necessary? Not at all. You would do just fine with the 7's, they are both great units. But it sure is fun and that state team discount is the deal!