I asked the Lowrance expert Darrell Rollins specifically the same question. Darrell's advice was stay with the external antenna unit 520c. I planned to mount this unit for front trolling motor use. You need to have the GPS antenna as close to where your sonar transducer is mounted to pin point where the coordinates are in relation to the sonar signal. For example if you sonar/GPS unit is 5 feet away from the transducer, you have a 5 foot error in regards to the GPS way points or sonar image displayed. The built in antenna units still have a few bugs compared to the external units. I'm sure Jim will have clearly stated explanation.
Tom
Tom