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.
Scythe is a Japanese company known for their high-quality fans and heat-sinks.
Like festerovic said the choice will come down to which one fits your mobo without interfering with your dimms and ram height