The X Series stabilizes all axis in one double Gyro.
It is helpful or a must to use the gyro stabilizer when you have to fight wind (which is always the enemy of cardanic stabilized equipment) or on any vehicle like aircraft, boats, cars, long-lense shots, etc. Steadicam operators often use 2 KS-6x6 gyros. The KS-8x8 is rather meant for heavy movie-cameras. The KS-6x6 is similarly different (weight!) from the KS-8x8. Both are same size, the wheels are both spinning at same speed (aprox. 20,000 RPM), same run-time off battery, and both work with the same inverter.

KS-6x6:
Weight: 6.8 lbs (3,1 kg)
Size: 7.64"(19,4 cm) x 7.64"(19,4 cm)
Starting: 4.6 amps (@12 volts) at start up dropping to 2.2 at full running speed of 20,000 rpm

KS-8x8:
Weight: 10.4 lbs (4,7 kg)
Size: 7.64" (19,4 cm) x 7.64"(19,4 cm)
Starting: 4.6 amps (@12 volts) at start up dropping to 2.2 at full running speed of 20,000 rpm

Inverter for single or double Gyro use
This 12 volt (rated up to 14.8 volt) Inverter is used to run either a KS-6x6 or KS-8x8 gyro. Size: 4.19" x 2.6" x 1.78" (10,6 cm x 6,6 cm x 4,5 cm)