Ultrasonic Scaler - Magnetostrictive
Used mainly in the United States, the magnetostrictive ultrasonic scaler uses a magnetostrictive transducer to create magnetic energy which is then converted to mechanical energy at varying frequencies.
Ultrasonic Scaler - Piezo
More prominant in Europe and Asia, piezo ultrasonic scaler vibrations are produced by oscillations of a quartz crystal in the handpiece.
Both technologies are considered to be very sound, providing consistent operation over a long period of time.
Repairs are typically minor, with most involving the handpiece or tips, which are easily replaceable. One additional consideration would be that most magnetostrictive ultrasonic scaler tips are interchangeable, while piezo ultrasonic scaler tips are typically specific to the manufacturer.
Pathogen Destroying Cavitation Bubbles?
Proponents of magnetostrictive scalers claim that their elliptical motion is more effective because it generates pathogen destroying cavitation bubbles 360° around the tip, while the piezo design vibrates more in a straight line creating the bubble effect only at the two ends of their back-and-forth cycle. However, the pattern of tip oscillation appears to be influenced far more by the geometry of the tip itself than by the design of scaler that 
powers it. For example, “S” shaped tips like left- and right-curving perio tips feature an elliptical vibration. But tips with a simple curve, like the Cavitron TFI® and Parkell Universal tip, are linear.
Independent researchers at the University of Kiel, and the Max Planck Institute reported that the vibration pattern of a magnetostrictive ultrasonic scaler seemed virtually the same as that of a piezo ultrasonic scaler.
DENTSPLY is considered the leader in this industry with the trademarked Cavitron units in use widely throughout the US. Hu-Friedy, a major dental instrument manufacturer, is now in the game with the SWERV3 magnetostrictive unit expected to be accepted by the industry with open arms. With hand instruments in most every office in the US, The Hu-Friedy name is synonymous with quality and workmanship.