UTM for me. Much easier to work with. Only UTM downside I see might be for large area navigation, such a airplane pilots need. Lat/Long is more common for those folks. With UTM, you can look at two gridlines (either NorthSouth or EastWest) and determine the distance between them in meters. Simple math. You can't do that with Lat/Long. The distance between lines of Longitude differ, depending on what Latitude you're at.