I am looking to buy a chartplotter/ fishfinder for my boat. I have it narrowed down between the Garmin 546S and the Raymarine A57D. Does anyone have any experiance with any of these units or companies? My boat is a 21' Steiger Craft with the pilot house. Raymarine said its unit has a built in antenna that can read through a hard top. What is your take on this? Would I still be better off having an external antenna? Thanks in advance.