I have a helix 12 with all the bells and whistles at the steering wheel and just put a base model helix 7 2d sonar up with Terrova to slab for white bass and hybrids under once fish are located on the helix 12 and spot locked over. The reason for this is that I cover lots of water searching for white bass and utilize multiple technologies on one screen to locate them but utilize very simple 2d sonar under the TM once spot locked on the school of fish. Sometimes I can get by with slabbing under the rear transom sonar but if the school is small enough, I may mark them on sonar and tell my terrova to "go to" and spot lock on the school and the back of my boat 24 feet away is simply too far away to catch the school.
Its a good argument that you may use your TM mounted sonar more than the helm sonar and would want it bigger, but I think the one with the bells and whistles for the majority of anglers would be at the steering wheel which is why the larger screen is more beneficial so more technologies can be seen at one time on a larger screen. The reason I can get by with a smaller unit on the front is because I'm only utilizing one technology on it (2d sonar) and not trying to locate fish but more see their reaction to my presentation and if they are still there.
Hope that gives some rational and things to think about! What would be awesome is to just buy 12 inch unit with bells and whistles for helm and 12inch base model for front but that would cost a bunch!
RespectTheFish on YouTube