Chances are good for most CPU's you won't see much of a difference between the two as both are great coolers, if I were choosing between the two I'd probably go more on price and pick the cheaper of the two.
However, with that being said, the Noctua probably has the slight edge in performance.
Both coolers allow you to adjust fan speed (Zalman through a wired controller with a dial, and Noctua through different fan adapter cables) so you can make either be as noisy or quiet as you want. The Zalman would be easier to adjust on the fly as the Noctua would require you to open up your case and change adaptors while the Zalman can be done on the go.
As far as quality goes, both Zalman and Noctua has both always performed well and we've never run into quality issues with either, maybe you could argue that Noctua might have a better fan, however, in my experience I've never had a Zalman or Noctua fan fail in any of our systems around here and would have no problems recommending either.
So basically for me it would come down to price and whichever one you could pick up for the cheapest....
Hope that helps, let me know if you have any other questions.
