The key difference between the two is the max boosting power offered.
The RG1000 is rated for 1000peak amps enabling boosting of gas vehicles up to 7.0L and diesel engines up to 3.5L. The RG600 is rated for 600 peak amps, perfect for boosting gas engines up to 6.0L and diesel engines up to 3.0L.
To allow for this extra power, the RG1000 features a thicker gauge cable for the clamps, upgraded INTELLIBOOST circuitry and a larger capacity battery. This larger battery also provides about 10% more charging capacity for USB/Laptops.
Other differences are the included accessories and packaging: The RG600 comes in a hard, plastic case and includes a 3in1 USB cable (micro, lightning and 30pin Apple), laptop tips, car charger and wall charger.
The RG1000 comes in an attractive cardboard box and includes a soft carrying pouch to make it easy to fit in a glove box or other small area. It also includes an updated 3in1 USB cable with lightning, micro-usb and USB Type C ends. As well it includes a car and wall charger.