If you want moving water in your bath, your main choice is between a whirlpool tub and an air tub. Though the two are similarly pampering, it helps to understand the basic differences.
In whirlpool tubs, it's the circulation of water--usually by built-in jets and hoses--that provides the massage. Air is mixed in, but water does most of the work. Air baths, on the other hand, force air through tiny holes to create an allover bubbling massage. No water circulates through the system. Air baths were first used in hospitals for their therapeutic effect.
With a whirlpool, a bather can usually adjust the air and water flow of the individual jets, or control their overall force at the pump. The number of jets in a tub can vary and isn't always the most important factor; power is important, too. A few powerful jets may move the same amount of water as several less-powerful jets. In an air bath, hole placement -- which is fixed -- determines the massage action.
Though less adjustable, air baths may be easier to clean. An automatic purge feature forces additional air out of the holes after the water has been drained. This clears the holes and also dries the air chamber and tub.
If oils and gels are part of your bath ritual, check with a dealer about restrictions on their use. Because no water is circulating through it, an air bath can withstand bath products without clogging. Most whirlpools cannot.
Finally, consider noise. Both whirlpool and air tubs can be loud. Expect 70-80 decibels, depending on how fast the motor and pumps are working. That's about the same level as normal traffic or a quiet train.