Both the Womanizer Duo and the Womanizer InsideOut have different intensity levels in the clitoral stimulator and insertable stimulator, but only the Womanizer Duo has different patterns.
The Womanizer Duo has a bulbed insertable shaft, whereas all of the Womanizer InsideOut's shaft is roughly the same circumference.
The Womanizer Duo has the Smart Silence feature, which means the vibrations and air pulse suction only kick in when the clitoral stimulator comes into contact with your body.
The Womanizer InsideOut measures 192 x 120 mm and the Womanizer Duo measures 88 x 203 x 58 mm