The specification that matters is rarely the one on the front of the box. Four things decide whether a product gets used twice.
Rumble against buzz
A low-frequency motor feels deeper; a high-frequency one feels more surface-level and numbs faster. Most people prefer the first and buy the second, because buzz is cheaper to build.
Noise
Anything above a quiet hum is audible through a door. If that matters, look for the decibel figure — a manufacturer who omits it usually has a reason.
Controls you can find without looking
Buttons you can feel apart beat a touch panel. A single cycling button means you cannot go back a step without going all the way round.
Charging
Magnetic charging is standard and sealed, which is what lets a product be fully waterproof. A rubber flap over a USB port is a leak waiting to happen.
Every product page here lists motor type, noise and charging, because they are the questions people write in to ask.