Tony, its mostly cost, but recording tracks, speed and tacking angles (and tracks, for those that record them) are big pluses for those persons looking to improve racing techique. The other difference is that the magnetic compass gives you 'heading' vs GPS device's'course'. For those units that have both, the difference between heading and course can be used (on long tacks) to determine how much the tide is knocking/lifting, or even facilitate understanding boat performance issues pertaining to alignment/foil lifting. Many GPS units will also calculate the velocity made good to a given target, which lots consider cheating, (but if you are staring at VMG on a catamaran in a buouy race, you are too deeply thinking about the boat, as the speed of cats makes that value rather hard to put into any real practice in a short length race). Many of the GPS units will also calculate distance to the line, which is definitely a contentious advantage 'issue' in big fleets, where the start can mean the difference in race outcome.