1. Enhanced Performance – Not specified, but presumably means you get priority in downloading data, so it might go faster. Possibly worth paying for, but the free version works fast enough for me.
2. Printing – A 40% improvement in printing quality (1000 pixels versus 1400 pixels) – This might be worth it. There are ways you can actually do the same thing for free, but they’re time consuming and may be a violation of the Google Earth Terms Of Service.
3. GPS Data Import (read only) – Hook up your GPS and download waypoint, track and route data directly into GE. Now that GE recently added support for NMEA, in addition to the standard Garmin and Magellan protocols it already supported, this is a lot more useful than it used to be. But there are other options for bringing GPS data into Google Earth that are free, and offer more flexibility. Not worth it.
4. Real-Time GPS Tracking – Puts an icon in Google Earth to mark your current position as measured by a connected GPS. A new addition, and reasonably cool, but there are better solutions for this that, if you’re lucky and have the right hardware, are free or cheap.
5. Spreadsheet Data Import – Allows you to import a .csv file with coordinates into Google Earth, and will do address coordinate lookup (geocoding) as well. But there’s a limit of 100 points in the Plus version, while free tools are available to both convert .csv files with coordinates into the .kml format, and to geocode address files and convert them into .kml files as well.
6. Advertisements - Mandatory in GE, optional in the Plus version. I could care less (or, I couldn’t care less).