I think it's all relative to your type of fishing. I'm kinda unique in that I run a stick steer and have my big unit mounted by my right knee. And bow unit right in front of me. I can run big unit on chart or chart and SI while running bow unit on 2d or DI. Most important to me is GPS on both. That way I can mark stuff on bow and transfer to main unit or vice averse. I mainly spider rig so that suits my purpose. If I had the opportunity I would run SI also.