The 10x provides either NMEA or PVT (Garmin) data output, so can be used directly with any navigation program. Also, being Bluetooth, you have a wider choice of where to place it.
The 20x only works directly with Garmin programs. To use it with other navigation programs, you would require GPS Gate to convert its output.
Garmin Mobile PC (the software) is identical in both cases and will only work with the included GPS (10x or 20x)
Terry