The question is too vague. With low noise fans, the Alpha works better because of its lower restriction design. With midrange fans, it's a tossup, and with fans that make your ears bleed, the slk 800 comes out on top.
The slk800 is also more susceptible to clogging in less than pristine environments because of the very narrow fin spacing... it fits more motherboards, and is a lot easier to install than the alpha...