An ultrasonic toothbrush is completely different that a “sonic” toothbrush. You can read about the difference online and on wikipedia under “electric toothbrushes”. An ultrasonic toothbrush gives off pressure waves typically using a piezo crystal imbedded near the bristles. The typical frequency of these pressure wave are around 1.6MHz, which translates to around 96,000,000 pulses per second or 192,000,000 movements per minute.
Sonic toothbrush work only on mechanical movement, typically around 31,000 movements per minute, which translates to about 258 Hz (not megahertz). It does not have any piezo crystals in it and therefore does not produce ultrasound.
Ultrasound travels well through water (saliva) and tears apart bacterial chains that make up plaque. I prefer an ultrasonic toothbrush that works both on ultrasound and sonic vibration at the same time. This means you get the best of both worlds.
Ultrasonic alone might not be as effective because it doesn’t have enough mechanical agitation to remove plaque. But ultrasonic could have the potential to break apart bacteria in a unique way. So ideally you would use both to get the benefits of both.