I currently have a helix 10di g2n mega at the bow and a helix 10si g2n mega at the console. Both are running individual 14 gauge wire to the battery. Being that it is winter I'm doing some upgrading. I'd like to upgrade the picture quality 'possibly' on my current units but also have the right wire if I upgrade them. Is 10 gauge 10/2 Marine sheilded wire what I should be using? Is there any downside in going too big? Also, should I run two sets of 10/2, one to each unit or sit the power off at the console? Then I wonder if I should add ferrite rings or chokes just to make sure. I'll obviously solder eveything. I'm trying to do it once and do it right. 

Increasing the conductor size won't have any effect on picture clarity. Shielded wire may help if you are experiencing interference. You can also try adding a choke. Humminbird might even send them to you at no charge.

 

14g should be plenty big enough.  Downside of going larger is cost, no measurable up side.  Graphs don't draw much current.

 

Keep splices out, solder any connections, and use dielectric grease in any plugs.  Conductivity losses (corrosion) is the main issue to worry about.