The Swiftech + fan will be more money, but it may cool somewhat better depending on what kind of fan you put on it. The Zalman is quieter, one piece, and you do not have to buy another fan. If you want the utmost performance, get the Swiftech with a high CFM fan. Otherwise, take the Zalman.
I've been trying to find direct comparisons between these two HSFs for a few days myself. I haven't been able to find direct comparisons, only reviews on one or the other, which really does nothing to compare the two.
Anyway the MCX64-V comes with a fan that is rated at 28dBA. The MCX6400-V is the one that comes without fan, and it's a little larger than the other one. According to this article here at [H]ardOCP, the MCX64-V performs quite closely to the larger one. The Zalman has a variable speed fan that is rated from 20-25dBA. So it seems like the Zalman is a bit quiter than the Swiftech. They're both priced similarly. $40.99 for the Zalman and $45.99 for the Swiftech on Newegg.
I'm not sure what motherboard you plan on using with this, but I plan on buying a MSI K8N Neo Platinum. Users of this board have reported that the Zalman will not fit on the board with the stock backplate, which they also say is a bit difficult to remove. Unless I can find a direct comparison soon that shows the the Zalman is significantly better than the Swiftech, I will be getting the Swiftech simply for ease of installation.