I always use the GPSMap 76S with a GA27C antenna when I'm hiking. On top of the backpack you get a much better reception, not only from the more powerful antenna, also your body covers less the sky. The additional current for the antenna is under 7ma (nothing to worry about). If you want to get everything out of an external antenna, mount it on a stick which slightly overtops your head and put it into the backpack. It works better than it sounds :-)
Pietsch