I spoke with my Kohler rep about this very thing. In particular, I asked about noise (which is supposedly lower with a water- cooled) and longevity. He chuckled about both.
He said that they may be a bit quieter, but not so much that you can tell it without instruments to measure the difference.
And regarding longevity, he said that they don't last longer than air- cooled.
You have to keep in mind that the kind of HWE heating technology driving the carpet care market right now, depends upon system components for heat. And the larger the component, the more BTU's available for heat transfer. THAT's what's behind larger blowers and larger motors... not longevity.
Longevity is something that's engineered- in. If a motor is used that has more power than needed, then it can, "coast" (i.e., run at lower RPM), so to speak. And in being able to coast, it will last longer.