I can't speak from experience, but I think they are very similar. Look here:http://www.frostytech.com/articleview.cfm?articleid=2215&page=5
They have tested these coolers (except they did the Zalman 9700, not the 9900; I can't see it making a huge difference) Look where they fall - in the top 6, and their numbers are pretty even.
The choice would be easy for me - which one fits, then which one is cheaper. I just checked for giggles, and the zalman was $75. Hmm. Seems pricey for the same performance as the $35 mugen. But the lightes may be worth it.