This is a common argument!
The problem is that some people don't mind noise while others do. That is really the cause of the big difference.
If you do not mind noise, you can tolerate relatively high-RPM fans on (in) your case and your case has a very good air flow through it. Then, you will not experience much difference between the silencer and the Zalman cooler. The excellent case cooling will make the effect of the heat dumped by the Zalman cooler negligible.
However, if you are a noise fanatic, you cool your case, and everything else in it, only as much as you have to. So, your PC components and your case will be warmer. This lets you drop the RPM of your fans, which minimizes noise. In this case, the heat dumped into the case by the Zalman cooler, or any other cooler that does not directly exhausts heat, will be quite noticeable.
Those who tell you that Zalman is better are often from the first group. Those who tell you that the silencer is better are often from the second group.