I have been a Sonicare user for decades. I decided to switch when Sonicare claimed it couldn't find my registration, so it wouldn't honor its warranty.
The AcquaSonic looks exactly like the Sonicare Diamond Clean, but costs one-fifth of the price ($36.95 on Amazon vs. $198.95). Incidentally, the "motor" in the Diamond Clean is identical to less-expensive Sonicare models. It only performs better because of the brush, which fits on the AquaSonic.
I have no idea how long the AquaSonic will last, but it has a 13 month warranty vs. two and a half years for Sonicare, if you register the purchase. So I can replace the Aquasonic and it would still cost less. Based on my experience with Sonicare, the wand will only stop working when the lithium battery dies or if you drop it on the floor. On the negative side, it isn't clear how you contact Beauty Imports (the seller of AquaSonic) if there is a problem while Sonicare's customer service (except for losing my registration) has been exceptional.
Addendum: Now that I have owned the AquaSonic for a few months, I can report that the brushing seems to be as effective as with a Sonicare (what counts is the brush, not the wand). On the plus side, I have been getting an incredible 60 two-minute brushes per charge (two a day for thirty days), more than twice my experience with Sonicare, suggesting that the battery will last longer. On the minus side, the timer stopped working after one week. Amazon eventually figured out how to fix the problem (Amazon handles tech support calls, but the rep doesn't specialize in AquaSonic -- she has to look up the answer). In case you have the same issue, just press the "on" button for 20 seconds, the same solution needed when Sonicare has this problem.
The Aquacare seems to clean just as well as Sonicare, is slightly (0.6 ounces) lighter, has a battery that lasts twice as long (four weeks versus two), and comes with more brushes (eight one). Sonicare comes with a pressure sensor, which I think is a solution in search of a problem.
The average AquaSonic review on Amazon (4.7 stars) is better than the $69.95 Sonicare 4100 (4.3 stars), with the bad reviews criticizing the electric toothbrush experience, the brush heads cracking, or the battery failing. The last two items are covered by warranty. If Beauty Imports isn't responsive, contact Amazon, which has incredibly good customer service.