If you want Side Imaging at the console and bow, it is best to have the sonar data where you are on the boat, not coming from a boat length away.
If you don't care about sonar accuracy, you can certainly share SI to a Sonar or Down Imaging model via Ethernet.
All my units are Side Imaging units. I "need" Side Imaging as it is my search tool and fishing tool no matter where I am on the boat. I add 360 to the mix to have Side Imaging when sitting still (it does the scanning while fishing).
MEGA Imaging is great for details and focused searching for "stuff". I don't use it for fishing due to the narrow coverage. I want to see "over there" to the edge of the Range setting.
The biggest benefit of a MEGA SI unit is its dedicated Down Imaging crystal in the transducer. I don't use DI for fishing (doesn't see "over there"), but some do.
Sort of boils down to how you would use the different sonar technologies and your willingness to fully learn how to use each.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, SOLIX 12 SI, 360, RC-1