Gerry
I have a 998cSI. It has 2D, DI, and SI. I use SI about 90+% of the time, with 2D and DI used to " fine tune " what I see in SI.
The amount of information you will see using SI compared to 2D and DI only will be amazing.
Learning to use SI isn't that hard. The obvious stuff is easy to see. With time you will be able to pick out things you missed originally.
It's like looking at an Xray. The info's there, you just have to know how to read it.
If at all possible, spend some time in a boat with someone that has a SI. You'll know the answer in less than a minute.
The #1 feature I would look for in any unit is the ability to record to a SD card. Play these recordings back in Humviewer and you will REALLY see what's there.
If that cayuga is Cayuga Lake in NY. ask at any good tackle shop. They'll know who has a Humminbird SI.
Although you may have answered your own question--- " About $300 difference is a lot for me to swallow".
The $2000. I spent didn't come easy, but I don't regret it for a minute.
Good luck on you choice.