Should I buy a GCV10 or install a 998C?

Looking for a little education......
I bought a boat that has a Garmin Echomap 70dv on it (7" display). I really like the touchscreen interface and clarity of it.
Garmin offers a GCV10 module that will make it have chirp frequencies and side imaging which I would like to have.
I also have a new Humminbird 998C HD SI unit that was a display unit at the local Academy sports.

Here's the issues. Do I sell the HB to get the funds for the GCV10 or install the HB on the boat? Since the GCV10 is Chirp, will it do a better job than the HB? The Garmin is flush mounted in the dash so I will have to cut the dash to fit the 8" display HB.

Please give me some reasonable advice based on performance of what I'm working with. In a perfect world, I'd sell em both and buy a mega imaging unit, but that's not in the cards right now.

Thanks,
Chris

I had a 70DV and a 70S plus the GVC-10, all linked together. I found it to excellent with one exception. Garmin does not show history. So if you are using 2D sonar and happen to notice some structure on the screen, you can't switch to DV or SV and see what you passed over. It will only show what's under you from the point you switched screens. I sold my 70S and GVC-10 to buy a Lowrance Elite 7Ti so I could link it to my Xi5 trolling motor. Truthfully I liked the Garmin better. I can't comment on the Humminbird you have. If you are running a MinnKota trolling motor and want to link up to a sonar, then keep the Humminbird. Here's some screenshots with the GVC-10Attachment 276265