Good Morning!
The Elite 7 HDI was a good little unit and never a seconds trouble and easy to see even in Florida Sun, but did not have the power to run some of the more modern chartplotter chips. That one also did not have side scan sonar which for my needs, is much better than looking down. Looking down in 2 foot of water is pretty useless

So as such, hard to compare the older 7 HDI unit to the much more advanced Elite 9 Ti which I now have
So I compared offerings from both SIMRAD and Lowrance (really the same company, Navico) and wanted a unit that had at least a few basic input buttons. The SIMRAD GO9 though a fine unit, is all touch screen and I heard more than one comment about wet hands and trouble with such. So the Lowrance which I am big fan, was clearly the more logical choice for my needs.
For you I would "Network it" and let the newer 9ti with it's much newer total scan transducer, be the main feed for all the units onboard. It can display both side and down at the same time (if needed) as well as have some other features your older HDS10 may not have. 1st thing I would do with the present HDS10, would ensure she is running the latest software and see if that brings online some newer features that "might" negate evening having to add the second unit?
If you do elect to go with the new 9Ti, then take the time to reach out to RMP (Russell Marine Products) and let Dee Dee know that Dave from Performance Outdoors sent you. I am quite sure you can get the best costing on it like she did for me
And when combined with no Sales Tax and free shipping, saved quite a few dollars at a even smaller base price nobody else was willing to match
You can find them located here:
https://www.russellmarineproducts.com/
And also here
Email:
[email protected]
By Phone: (316) 313-4113
Here is a video in the differences on the various Transducers for the Lowrance line from Justin at RMP, which may help clear up what you may actually need for your best use case
And don't forget to view this on how to set up a NEMA 2000 Network so you can pull all those units together
https://www.performanceoutdoors.net/threads/how-to-build-a-lowrance-nmea-network.9283/
Hope this helps and call me if you have any questions

Dave